NOVICE PAINTING THOUGHTS

After my paint fiasco, I decided that I couldn't afford the frustration of dealing with a contract painter ... it was driving me crazy and there was only one solution ... buy my own paint gun(s). I've done a lot of painting through the years using various spray cans, brushes and rollers, but only very occassionally using someone else's gun a long time ago. I actually chose to get two guns; a detail gun for the smaller parts and an HVLP gun for working on the larger panels. As my experience grows I'll try post some mistakes & tips that I've learned. Please keep in mind that this is a learning process for me and I'll gladly add tips from those that are more experienced.

For those that are still leading up to this phase, I can say that this was not as difficult a task as I had assumed. Sure it's extremely time consuming, but it is much more convenient to be able to paint the various pieces without relying on others and one should be able to achieve a reasonable finish with a bit of practice. If one is still unsure of their capabilities, there are lots of smaller parts that are less obvious and can be done first so as to get some practice. One may still want to consider contracting out the frame, boom and final cab painting as these are the most difficult and most obvious areas.

LOCATION
EQUIPMENT and SUPPLIES
PAINT
TRICKS

LOCATION

Before committing to spray painting, the first decision is to decide where it will be done. We'd all like to have free and generous access to a professional spray booth, but reality says that only a few very lucky builders will have this privilege. I haven't seen electrostatic spraying, but the various detail guns, HVLP guns and other types *WILL* create a lot of overspray. One can assume that this will get on everything and everywhere that is not completely covered or masked. The other important consideration is ventilation, both for the removal of overspray and more importantly, for fresh breathing air.

One option is to do the spraying outdoors, but that makes one dependant on the proper weather conditions of temperature, wind and humidity. Around here, early morning and late evening are usually best for the wind, but the humidity and temperature can be wrong. During our winters, there might be a day or two a month where outdoor painting is just viable but the parts would probably have to be brought inside immediately for drying and curing.

A garage might be viable, but usually with limitations. Firstly is the overspray problem which can be partially handled by getting a roll of polyethylene (vapour barrier or a lighter duty style) and draping the walls, floor and anything else that needs protection. The bigger problem is ventilation. I have a forced air heating duct into my attached garage with no return vent back into the main house. If I'm doing any painting in there, I also open the vehicle door a bit which creates a gap at the top and the bottom which helps to circulate the air, especially if there is a bit of a breeze outside. Still, I'll only do a small batch of parts at one time and spend minimal time inside during the actual spraying process. In the winter, due to the cold the door can't be opened and I'll only do one or two parts at a time if necessary ... hardly productive, but good for a quick need-it-painted-now part. Note that a very good mask is absolutely essential when doing this and a forced air mask such as the HobbyAir unit would be the preferred choice for most of the epoxies.

One of the nearby hangar owners constructed a "booth" using 2x2 framing and polyethylene. There is an intake air filter and just a household fan used to extract air through a vent to the outside. It certainly wouldn't pass a safety inspection, but it is useable for some of the larger parts. Again, one of my biggest concerns is the fresh air availability and while this system works, it doesn't extract the fumes and overspray nearly as quickly as a proper booth.

Regardless of location, the other issue is the ability to hang parts for the actual spraying and also for the initial tack-free drying time which for many of the epoxies / urethanes is 8+ hours. With my schedule and paint drying / re-coating / curing times, I find that I can do three or maybe four sessions a day, and the parts with final topcoat are moved (still on their hangars) just before the next session into another area to complete their cure. My preference is to not move the topcoated parts for at least two hours. Coat hangers, safety wire, picture wire, etc. can all make good hanger material but one needs some kind of hook or rack arrangement to hook them onto, both when painting and when drying.

Another key issue is the availability of good light. I've tried to spray paint at various times under different lighting conditions but unless one is very talented, good light is essential, especially on the final topcoat. The primer and initial topcoat aren't as ddependant on this as one can see the colour change happening and judge the amount of paint that is being applied. With a second topcoat, or the final passes on a single topcoat, the problem becomes one of deciding when you have truly applied a complete "wet" coat ... too little paint and one won't get the gloss from urethanes, and too much leads to ugly runs. I've been trying to stop spraying just after I get a consistent looking wet coat and so far I haven't had any problems with runs. I think my paint will allow a bit more buildup than this without runs, but I'm not about to experiment on an actual part.

One of the big non-no's around painting and a way to ruin a perfectly good job is dust. There are people who go to the extremes of wetting down the floors and possibly the walls but I won't go that far and I think one can introduce new problems such as humidity. I find that if the part is properly prepared and there are no strong air currents, then I'm fine so long as I'm careful not to stir up excess overspray and dust that may be on the floor. For the garage painting, I partially open the door long before I start painting so that any dust that is stirred up will have a chance to settle and then just before I start to paint I go over the part lightly with a tack cloth. Although this technique isn't perfect, it seems to give reasonable results for me ... certainly not perfect, but without too many "nasties". The biggest problem I have is with the intercoat time and the fact that new dust gets a chance to be stirred up and settle on the part. It seems to help a bit to first shoot a bit of straight air onto the part before a recoat, but there's no question that I get more "nasties" than if I was using a proper paint booth with filters.

EQUIPMENT and SUPPLIES

Besides the actual paint(s) and thinner(s), there are several other items required before one starts painting. I'm sure this list isn't exhaustive, but it will give a starting reference and some ideas.

• Compressor - Pretty obvious, but I thought I'd add a few comments. I'd highly recommend that one adds a good particle / moisture filter to their air system if it's not already installed. Most paint guns have a CFM (Cubic Feet per Minute) specification for the compressor, but I have no idea where they come up with these. I can only assume they're based on the gun adjusted for it's maximum air and the trigger being held down continuously. I know my compressor has about 50% more CFM rating than my one gun, but it only kicks on maybe once or twice during a session on smaller parts. If the gun itself doesn't have them, the air system should have an adjustable pressure regulator and a gauge. I also find it easier on my arm if the air line is either one of the very flexible ones or there is a multi-way swivel at the base of the gun.
  
  
• Paint Gun(s) - Since I have both a detail gun and an HVLP gun, I'll deal with them seperately:
  
  Detail Gun: I find that this gives excellent results on smaller pieces and on round tubes without too much overspray. When using this gun, I have my compressor set for about 30-40 PSI and the air pressure adjustment on the gun set quite low. The paint volume adjustment was then set using various test strips (actually the plastic on the wall). The downside is that the way I have my gun adjusted, I only get a really effective pattern that is about 1" wide. I've gotten used to these settings and find that I'm quite comfortable using it on smaller parts, but it really isn't practical for large surfaces.
  
  HVLP Gun: I finally got a chance to do some more testing with this gun and so far I still can't get the results that I want. It could be the weather as it was just warm enough to do some spraying and soon after I started there was a front/buildup decided to roll in and the temperature never really increased, but the humidity did. While doing interior hidden areas and primer, I've tried a lot of permutations of the various settings plus thinner but couldn't seem to find one that gives me either a fine spray or one that lays down a nice uniform thin coat. Although the weather was probably a big contributer, I seem to be on the edge of getting a fine orange peel most of the time unless I lay down a really wet coat ... unfortunately I was working with vertical surfaces and afraid of runs.
  
  With some warmer weather, I've done some more testing / painting with this gun and can now sometimes get just acceptable results with it. The one real advantage over the detail gun is that the painting process goes much quicker because of the wider pattern. However, I still can't get as fine a spray as I'd like (i.e. like the detail gun) and I believe this is causing the difficulties laying down a smooth coat. Perhaps this is why there are different nozzles available and also guns that cost a LOT more. After using this gun again, I've come to the conclusion that this gun, my paint and my technique just don't work together ... period. Without being able to get the fine spray, despite a lot of adjusting / testing, I consistently get some degree of orange peel and the results just don't justify the ease of use. I regret not discussing the gun purchase decision with the paint store personnel ... they've been extremely open and honest and I'm sure they could have recommended an acceptable gun at a reasonable price. I eventually just sold this gun.
  
  Primer Pistol: I also have a Primer Pistol which I thought I'd give a try since I hate the painting cleanup process. The cleanup is certainly as advertised (i.e. throw away film canisters) and while it does work, I certainly wouldn't do any amount of topcoating with it. The biggest problem I found is that I couldn't get a consistent thin coat. I reduced the air pressure to well under half the recommended pressure and still found that I had to hold the part about 1-1/2 to 2 feet away in order to prevent a heavy buildup / runs. Perhaps more practice and experience would result in a better pattern and coverage, but for now I'm only using it on very small touch up jobs and hidden pieces.
  
  
• Gun Holder / Hook - Again, one of those obvious things but easy to overlook the first time. This is probably more important for a gravity fed gun since once the cup is filled it must remain upright, yet the design precludes doing this without a hook. Even a long nail will work, but make sure to test it beforehand and feel comfortable that the gun will stay put.
  
  
• Respirator Mask - As a bare minimum, one should use a good mask that is rated for organic solvents. Mine also has a pre-filter that allows for double use as a dust mask when sanding. A full face mask will protect your eyes and would be very handy for those of us that have to wear glasses (luckily I only have to do this for detail work). By far the best solution would be to use a HobbyAir system with a full face mask.
  
  
• Gloves - No matter how careful I am, even with just a spray can, it seems like I always get overspray or cleanup solvents on my hands. Since most of these solvents are not very healthy, I prefer to always wear gloves. While just disposible latex gloves will work fine for the actual spraying, I prefer to use nitrile gloves which stand up much better to the solvents during cleanup. As a sidenote, both types are also available in the cleaning section of my local 24-hour grocery store in case I run out of them at an odd time.
  
  
• Mixing cups - Since I'm using an epoxy urethane paint, it's necessary to mix the two parts and also to add a bit of thinner. My local paint shop had some great clear plastic cups that are well marked with graduations and make this a simple task. For most of my work, the smaller 16 ounce cups are fine, but there are also larger ones available. I also have some of the one ounce graduated cups, commonly used for dispensing medicines, that I bought in quantity (100 for ~$1.50) at a medical supply shop. In addition to being used for mixing small batches of primer, I find that I use these small cups and a pair of hemostats to dip into the paint cans rather than just pouring and filling the can's rim.
  
  Tip: When mixing very small batches of primer or paint in the one ounce clear cups, I find it somewhat hard to read the graduations when adding the two components. I find that if I first use a sharpie pen to make a small mark at the two required levels, it then makes it much easier to add the right amount of each component.
  
  
• Mixing stick - A stirring / mixing stick will be required and the metal ones make for a simple solvent wipedown and ready for re-use. I didn't have an appropriate piece of metal handy and chose to just use a square-ended chop stick since the bamboo really doesn't absorb much moisture and is easy to clean up. The best part is that I'd bought a pack of something like twenty of them in the local discount store for $2. Around the hangar we use tongue depressors for mixing epoxy resins and these also work well for mixing paint.
  
  
• Filters - When pouring the paint from the mixing cup into the gun, one really should use one of the disposable filters that are available from the better paint stores. Since I only have the large filters, this is somewhat difficult with my detail gun and I haven't always been doing it. So far I've been lucky, but I'm also very careful to only use very clean cups and mixing sticks while also dipping the paint from the cans.
  
  
• Tack cloths - No matter how careful one is with preparation, it is probably a good idea to give the surfaces a quick once-over with a tack cloth after the part has been hung and is ready for spraying. I've tried a white version and the yellow ones (Bondo) and find that the yellow ones are slightly less "waxy" and easier to use. The cloth should first be unfolded and then just gently bunched into a ball. This is then just gently wiped over the surface ... too much pressure and it will transfer the sticky residue which may cause painting problems.
  
  
• Clean Up Solvents - The handiest solvent I've found for this is Gun Wash ... I've used thinners in the past, but I now much prefer gun wash. I put about an inch of it in the bottom of an ice cream pail (with cover) and at the end of a paint session use it to clean up my gloves, the gun, mixing cups and anything else that needs it. Even though it gets discoloured, this can be re-used several times before it needs to be refreshed. I find that if I wipe up the cleaned surface with a fresh piece of shop towel while the gun wash is still wet, then any residue is easily removed.
  Update: I had to get some more of this and it was obvious that it was a different formula even though I bought it from the same supplier. Instead of thinning the paint for removal, it actually caused it to gum up ... one more issue to deal with. I've subsequently talked to the paint store and they recognized that they changed suppliers and the "new" gun wash was not acceptable.
  
  I also keep a small container of MEK (Methyl Ethyl Ketone) handy when I'm painting as it seems to be a very powerful universal solvent for the various products that I use. Rather than a bottle, I find that a bit of it in an old margarine container is extremely handy for either dipping small parts, rags, etc.
  
  Sidenote - I'd got some overspray on my glasses and finally figured that I had nothing to lose so I tried gun wash to remove it. It worked like a charm without any apparent harmful effects. Although it worked for me, I'm not going to be responsible if you try this and it ruins your glasses. I've also tried using 99% Isopropyl Alcohol to clean my glasses and this also works ... I think I'll stick with this as it's far less agressive and I have no issues about using it without gloves.
  
  
• Masking Tape / Materials: The amount of masking required is obviously related to the complexity of the paint scheme, but it's a fair bet that some will be required. For the actual edge, I prefer to use the special narrow and thin tape that is specifically designed for edging. The main masking using either just painter's masking tape or a combination of tape / material is then layed on top of this, but slightly back from the edge. Large areas can be masked using special painter's paper or plastic, but not something like newspaper which is porous and will bleed it's ink. I've actually used a double layer of some old computer fan-fold paper with good success.
  
  Tip: Removing the mask while the paint is still relatively wet will help to smoothen the edge and prevent cracking/peeling. If one is masking an awkward area that is large enough to require the use of paper or plastic, then a little pre-planning for early edge tape removal will make it much easier. I've found that trying to remove large masks with wet paint invariably leads to an oopsie where the mask touches the paint or residue gets on areas that weren't painted. By moving the main mask to just a bit away from the edge tape, an extra strip of masking tape can be used as a bridge between the two. After painting and while the part is still wet, first the bridge tape is removed and then the edge tape, but the main mask remains until the paint is totally tack free.
  
  Note: Although a riveted seam may seam solid, it's amazing how overspray can work it's way through any slight cracks. If one is concerned about the appearance on the other side, they may want to try putting some masking tape along the backside of a seam. A good wet-riveted seam should alleviate this concern.
  
  
• Excess Paint and Solvent Disposal - There is going to be some excess paint and contaminated solvents that will need to be disposed of. I suppose it would be possible to just place them outside and let them set or evaporate, but I don't like to do this. Instead, I keep a one gallon wide-mouth plastic jug with lid nearby and place any of the unwanted materials in there. This can eventually be disposed of at either a proper disposal site or perhaps through a local paint shop ... then again, while not environmentally friendly, for those of us on the Prairies there's always the option of trying to paint a gopher while it's in it's lair.

PAINT

I've been primarily using Endura products which are an epoxy urethane and I'm sure many of these points are relevant to other urethane paints. If nothing else, they may spark some questions about the product that the reader will be using. I have used a bit of other paint, namely a spray can of Zinc Chromate primer and some of the Krylon Epoxy Enamel spray cans. The thing I don't like about both of these products is that they're not chemical-proof when they're fully dry/cured ... if you don't believe me, then just try doing some MEK wipes on a fully cured test sample. The positive thing about these products is that they're easy to use with no mixing and only minor cleanup issues.

The zinc chromate spray can was used for a few areas, such as installing nutplates that were being coated just before riveting, since this is a very quick drying product and doesn't really slow down the building process.

The Krylon Epoxy Enamel was used on a few areas, such as the floor board pan exterior surfaces, which should never see abrasion or chemicals. While I can get a nice shiny thin uniform finish with this product, the one thing I don't like about it is that it is very "soft" for several days. In fact, I found that I try to leave it for at least a week before actually using the part.

The other kind of spray "paint" I used is Perma-Slick G. While technically this may not be a paint in the classic sense, it is a MoS2 based solid film lubricant in an epoxy binder that is applied at room temperature and useable after 1/2 hour with a full cure in 24 hours. I used it in areas where there were sliding surfaces, such as in the collective and cyclic controls. It was applied over well cleaned bare metal and goes on as a dark gray colour and produces a very slippery surface that turns silvery when buffed. It has a Mil-Spec (MIL-L-23398, and MIL-PRF-46147C) and is supposed to be very durable ... time will tell. Note that even the overspray is both fine and slippery, but it can be cleaned up with MEK and probably other such solvents. Also, it should go on as a matte gray and needs to be occasionally agitated when spraying to keep the solids from settling. The actual "paint" is very tenatious and I'm led to believe that it only requires a very thin coat (.0003" to .0005") to maintain it's lubrication and protective qualities. Also, since this is a lubricant I've been careful to try not cross-contaminate unpainted pieces and work surfaces when working with Perma-Slik G ... I haven't tested other paints on top of it, but I highly doubt that they'll adhere very well. I apply this product as the very last paint on a part and also use a unique pair of gloves that are not used for any other purpose so as to try avoid any transfer of MoS2.

I had read some articles about the use of a protection coat for anodized surfaces and I obtained a small can of Everbrite early in my build process. After carefully reading all the instructions, this was applied to my anodized oil tank cover plate using the described wiping method. I got a lot of streaking and wasn't impressed with the result ... consequently I didn't apply it to any more pieces. During final assembly, I had to decide what to do with this piece ... I ended up scuffing / sanding it with maroon Scotch-brite and then spraying on another coat of Everbrite. This turned out extremely well and if I'd seen this result on the first test then I'd probably have sprayed all my anodized parts this way. On the other hand, I subsequently messed this piece up again by exposing it to acetone as I was wiping off some of excess pipe sealant ... I think I'll stick with my epoxy urethane, including clear.

Primer

My primary primer/sealer has been Endura EP-2C which is a two-component epoxy urethane that contains Zinc Chromate, dries to a medium green colour and is mixed in a 1:1 ratio with no induction time (i.e. delay from mix to useage requirement). For spraying, I usually add about 15% - 20% thinner and for brushing I just use it without thinner. Because it's an epoxy, one has to be careful about the topcoat timing, otherwise it will need to be scuffed or sanded to obtain good inter-coat adhesion. Although this window is up to 24 hours, I always try to spray the topcoat within 12 hours; on a thin single coat of primer it can be as low as 1 hour, whereas on a double coat of primer it should set for 8-12 hours.

I believe that many times we apply a primer much thicker than actually required. It has been explained to me that on a properly prepared surface, primer is basically used as both a sealer and to provide good adhesion between the substrate and the topcoat. Only a very thin coat is required to achieve this and I have noticed that many factory aircraft only use a very thin coating on aluminum, which I have heard described as "transparent", to provide protection. I probably still lay it on a bit thicker than this, especially on steel, but I'm trying to get just a thin even coverage ... I still have lots more parts to practice on.

Another alternative for a primer would be a "self-etching" primer. This would certainly speed up the finishing process by eliminating the need for alumiprep and alodine, but obviously the final result wouldn't be as corrosion resistant since there is no conversion coating within the metal. Also, one needs to carefully check the compatibility of their primer and any topcoat that may be used. I haven't personally worked with these, but they do seem to be popular with the Van's RV builders.

Topcoat

My primary topcoat is Endura EX-2C which is a two-component type epoxy urethane that is mixed in a 1:1 ratio with no induction time. Again, I usually add about 15% - 20% thinner for spraying. At this time, my normal coating consists of a mist coat or two followed by a fairly full coat but perhaps just before or at the full "wet" stage. This is allowed to dry for a minimum of 1/2 hour and I then give it the final full wet coat. So far I've been quite happy with the results as I seem to get a full wet-coat looking finish without excessive buildup or orange peel. When I looked carefully at some pieces done by a "professional", I noted that he put the paint on much thicker but it was just starting to orange peel and in some places caused some fit issues upon re-assembly of the parts.

It's interesting to note that different colours tend to accent problems differently. Ultimately, the final "wet" coat will determine the finish and if it has been applied quickly and uniformly then I find the finish totally acceptable. The one thing I've tried to avoid is dry spray and overspray at this stage as it will show up as a dull bumpy look, sort of like a dull fine sandpaper. The other potential problem which can be be caused by problems on any of the layers, is orange peel. It's my understanding that this is primarily caused by too much pressure at the gun or possibly from too much overspray (especially with super catalyst); trying to lay down extra paint to smooth it out will not work in general and will probably only make the situation worse. The individual gun also seems to affect orange peel results and I believe this is sometimes due to the relatively thick nature of urethane paints and not using the proper tip in the gun.

I chose to not go with a clear coat and this was primarily due to the potential weight buildup. If I was starting over, I would certainly re-think this decision. On very solid colours, like white and black, I don't think it would make much of a durability difference. However, the red that I used is very susceptible to showing scuff marks. If it is scuffed but not scratched through, it shows up as a very dull orangeish area that is hard to rub out without applying more fresh paint. The other effect of a clear coat is to give more of a reflective finish with a "deeper" look to the colours. The best way I can describe this is glass-like versus plastic looking without the clear. It also tends to smoothen colour separation lines. Certainly if one is trying to build a "show special" they should consider clear coat(s). I ended up using a little bit of clear ... semi-flat on the instrument pod and gloss on a couple of pieces where I had to scuff then for a paint mask.

Special Additives

The product that I use has a super catalyst and a retarder available plus slow, regular and fast thinners. While most painters will tend to use the super catalyst to speed up the process, I believe this can easily lead to orange peel and dry spray if one is not very used to working with it. I discovered that the original application for the super catalyst was for pin striping applications and not in basic painting. I tend to be conscious of the weather conditions and not use any additives except for the regular thinner. The one place I might consider additives would be on painting the frame / boom where the use of the slow thinner, and possibly a bit of the retarder, might allow for more working time and lessen the effects of any overspray.
Update: I've now used the retarder due to hot weather and the more I use it, the more I like it. In general, the retarder allows a longer time for the paint to flow out thus preventing orange peel and I can't find any difference in the hardness or similar qualities of the cured results. I've now started using some retarder all of the time.

Tip: I've heard it mentioned in several articles that the best source of painting information is the paint shop. My supplier is a specialty shop that primarily deals only in the product that I'm using and I have to agree that I have received excellent and very helpful advice. If I have questions, I tend to go there in the late morning to mid afternoon timeframe since this is their quiet time and they will take a lot of time to fully answer my neophyte questions.

TRICKS I've Tried or Learned

Coating Inside Surfaces of Tubes

There are several open-ended tubes in the cyclic and collective controls plus the horizontal stab and I was worried about getting some corrosion protection onto the bare steel inner surfaces. These were first cleaned using maroon Scotch-Bright on a gun cleaning rod (see Tools section). I've also tried a brake hone to remove the internal mill scale and it also works very well. I tried test spraying into very short tubes and all I succeeded in doing is proving that I can create runs without an even coating. For short tubes, especially closed-ended ones such as the skid feet support stubs, I'm now using a brush on them.

For longer tubes, the painting solution I now use is to first tape off one end of the tube and any cross-bolt holes. At the end of a primer spraying session for different pieces, I usually have a bit of mixed primer left and I then pour about 1/2 to 1 ounce of it into the tube which is held vertical with the taped end down. The tube is slowly turned on it's axis while at the same time slowly lowering it from vertical to horizontal. This ensures that all inner surfaces get a good long soaking in the liquid primer, but one has to be careful as the tube reaches horizontal to get all the areas around the opening. Once it has been fully coated the remaining primer is poured out, the tape on the capped end is removed and the tube hung vertically to dry. Note that there will probably be some solvent cleanup on the outside of the tube required and that it will drip for several minutes as the excess primer continues to run out. Also, there will be some primer coming out as the tape is removed, so be sure to do this over a rag or some other cleanable surface. I did some tests using small mixing cups and was amazed that most of the primer is actually recovered so it would appear that the tube gets a thorough coverage without too much excess buildup. For those that really want extra protection, the process can be repeated using the remaining paint from the next topcoat session.

On the landing gear legs, I used a flapper wheel on a 12" drill extension to first clear out the mill scale. As a variation of the above technique, I found it best to actually tape off both ends of the tubes before rotating them to spread the primer. This worked especially well on the rear legs which are "bent".

Masking

There are several pieces that I masked so that I could apply multiple colours on them or both my standard urethane paint and also Perma-Slik G in selected areas. I have a roll of special 1/4" wide thin flexible vinyl tape that I used to do the outline and then used green masking tape and paper/plastic as appropriate to complete the mask. The special masking tape allows a sharp edge and does not allow any edge bleeding the same way most regular masking tapes do. If possible, the masking should be removed relatively soon after painting which will help to smoothen the edge. While the full cure interval for my paint is 7 days, the basic drying time is about 24 hours and I found that anything much over that meant I had to be very careful removing the edging tape. For those that haven't done this before, the masking tape should also be removed by essentially folding it back over itself as one is pulling it off.

My epoxy urethane paint is not effected by MEK once it's had it's full cure. I found a few areas where I was applying Perma-Slik G that were quite time consuming to do the final detail masking on and instead of spending a lot of time to do this, I just left small areas (perhaps 1/8" or so) unmasked. Once the part had been sprayed with Perma-Slik G and it had dried for 1/2 hour or so, I then went back and used MEK on small pieces of shop towel to wipe off the excess. Not the classiest way of doing it, but it seemed to work well for me. One just has to be very careful not to wipe off the Perma-Slik G from the areas where it was really meant to be. Note that this technique becomes much more difficult if the Perma-Slik G has had a full cure since it becomes more resistant to the MEK.

Masking can be extremely time consuming, but any openings will allow overspray to find them and get onto the surface below. There were a couple of pieces, such as the ground wheels, where I wanted to selectively apply a second coating using a different colour or product. I chose to just mask the separation line and then use a cheap extra large freezer bag to cover the bulk of the part. A hole was cut where the part would exit and after the part was inserted it was taped around the opening. The rest of the bag was just wrapped around the part and a couple of pieces of tape would seal it and hold it in place. Really quick to do and the $0.15 for the bag was well worth the time saving. I've also been known to use this same kind of technique on larger parts using a garbage bag.

Note: In general, plastic (generic term) does not make a good material to make masks from. For some reason, it actually attracts overspray (perhaps some kind of electrostatic attraction) and it will find even the smallest opening to get onto the underlying surface. Ironically, I found that polyethylene did make a reasonable protection for the garage floor and walls since it seems to attract the overspray and stops it from staying airborne.

Sanding versus Masking

Since masking can take a lot of time, it's sometimes tempting to not mask on the initial main or base coat and instead to just sand that area off when applying a subsequent trim colour or different paint. If there's only a bit of trim colour and thickness is not an issue then it probably is easier to sand for adhesion than to mask. However, for larger areas or areas that will need to be sanded down to bare metal or primer (such as for subsequent coats of Perma-Slik G), then I believe it's usually easier to mask the area off when doing the initial base coat. A good urethane epoxy takes a lot of sanding effort to remove versus just taking a bit of time and using a bit of masking tape.

Hanging Parts "Spinning"

I hang all my parts for painting using thick safety wire or heavy picture frame wire. One of the problems with hanging some parts is that they will move and spin when they're hit with the air pressure from the gun. Obviously one of the solutions is to place them on a chicken wire frame. Otherwise the solution is to either try time the spinning to the pulling of the trigger (very difficult to get good coverage) or to stop the movement. If there are extra holes in the part, one solution can be to use a piece of hooked wire held in the the other hand and temporarily placed into one of the spare holes to lightly pull on the part as one is spraying. There were a couple of parts where I actually drilled a 1/16" hole in hidden areas so that I could hang the part properly.

I tried using the chicken wire frame for several small pieces and it worked well for pieces that are not very visible after assembly. However, there were two potential problems. The first is that there can be a slight buildup of paint where the piece rests on the frame and the second one is overspray. I placed the chicken wire on a very large box that had a lot of large vent holes cut into it but I still noticed that there was a fine dusting of overspray on the backside of the pieces. This could be okay if the final coat is on the visible side but otherwise I think there needs to be either a very open support structure or some kind of suction within the box. I've resorted to hanging all pieces for painting.

Identifying Similar Pieces or Orientation After Painting

There are several pieces that are essentially duplicated for the left and right side and custom fit during fabrication but prior to paint. My issue was how to figure out which was which without a lot of trial and error after painting. For pieces like these, I've made it a habit to find an obscure area on them and use a file to place a small notch on the piece that goes on the right side. In the event that I also need to figure out top from bottom, I place one small notch in the bottom of the left piece and two small notches in the bottom of the right piece. The other option on flat pieces is to just start a hole with a drill such that it makes a noticeable mark but doesn't go anywhere near through the material. After painting, it's still very easy to identify which part goes on which side and in what orientation. I'm sure someone is going to wonder why I mark the right side with more marking than the left side ... I simply needed an easy way to remember it and the letter "R" is "bigger" than the letter "L", or perhaps it was because I was trying to keep the pilot's side more pristine. Regardless, it works for me and I've adopted this convention.

Spray Can Nozzles

The various types of spray paints always talk about inverting the can and spraying till the nozzle is clear at the end of a session. I used to do this and it always seemed like such a waste of product, especially after just a really quick spray of a small part or two, plus it can make an extra mess, overspray, etc. to clean up. I've now changed my approach and just use a small piece of shop towel that is well soaked in the appropriate solvent and thoroughly wipe the nozzle clean until I can't get any more residue from it. In the dozens and dozens of times I've done this with various spray paints, I still haven't had even a hint of a clogged nozzle the next time I've used the can.
Update: I now have to admit that I had one nozzle that became clogged. This was on a nearly empty can of primer on which I had previously used the above trick dozens of times.

Rag Disposal

I have a phobia about fire and always worry about such things as solvent soaked rag disposal. While doing painting, and it's preparation, I find that I use a lot of shop towels soaked with various solvents such as MEK, isopropyl alcohol, acetone, gun wash, thinners, etc. but I try to use as small a piece of towel as appropriate. I keep a cardboard box nearby and as each piece of towel is used, it is thrown into the box, preferably hanging from the rim. At the end of the session, this box is placed outside and the various solvents seem to flash off completely in a very short time. I find that the fully soaked gun wash rags take somewhat longer to flash off, so I sometimes just place them in the wide-mouth 1 gallon jug that I use for excess paint / solvent disposal.

Painting the Boom

The boom has a lot of tricky corners that one has to try paint. We hung my boom right side up with the top slightly below shoulder level and it did allow reasonable access to the various areas. Then again, my painter made a mess of it so I don't know if this is the best way. I've since talked to several builders who have built a "rotisserie" jig to hold the boom and allow it to be rotated as it's being painted. I can certainly understand how this would make it much easier.

Small Batches of Parts

I've gotten into the situation a couple of times where I really only want to spray a couple of small pieces and I'm still within the epoxy recoat window. In particular, this can happen when I'm using the chicken wire technique for small parts and both sides need to be painted. It takes a long time (12 - 24 hours) before I'm comfortable flipping the part over onto it's newly painted surface and it's really tempting to just keep any leftover mixed epoxy from the previous batch and then thinning it a bit and shooting the parts once I'm comfortable flipping them. The downside is that it takes 5 minutes to spray the parts and 15-30 minutes to properly clean the gun, the mixing cups and all the other related stuff. I've found that if possible, it's easier to wait until the next time that I'm spraying that particular colour and just include the flip side of the pieces in the next batch. It only takes a couple of minutes to scuff or sand the pieces for proper intercoat adhesion versus the mess of cleaning everything up for a small batch of parts.

The other problem that I've had with some small parts on a wire frame is with overspray. I tried placing the screen on top of a large cardboard box and even put several holes (about 4" round) in the box to try vent the overspray. Obviously I didn't put enough (or large enough) holes in the box as I've had a few parts that got some overspray on the backside. The ultimate solution is to mask the parts, but as a minimum I will be taking care that if there is a visible side on the part then it will receive the last wet coat.

Brushing

So far, everything that I've talked about is for spray application of paint but the epoxy urethane that I use can actually be brushed quite effectively. This is much more effective on small parts where one won't see any inconsistencies or ripples in the coating or when just doing a small touch-up. I've tried using various brushes, from foam to acid types, and they can all be used with the difference being the amount of paint they lay down per pass and the amount of brush marks they leave. I've even used Q-tips to put primer onto flush rivet heads that were installed on a pre-primed panel. One can be creative with the main concern being the topcoat and the final preparation for it. Spraying still seems to leave the best final topcoat finish for the least effort, especially on larger pieces.

I brushed a topcoat onto a lot of the hidden tubes on the main cabin frame. This was quite successful (but time consuming) and I learned a few tricks. My paint should not be thinned when doing this and it actually becomes much easier to work with after it's been mixed for an hour or so as it becomes even thicker and one coat has very good coverage and quality. I added a bit of retarder to the mixed paint and this really seemed to help the brush marks smooth out and blend in. Foam brushes work extremely well, but the solvents/paint actually decompose the foam ... after about 15 minutes to 1/2 hour or so they have to be thrown out and replaced. Foam brushes retain a LOT of paint and it's best to try squeeze some of this out before using them to apply the paint.

Rolling

Fine foam rollers can be used to apply paint, but I find they tend to leave a somwhat "coarse" finish unless the coat is quite heavy. The one place my hangar mates use rollers is for the first coat of high-build primer on composite surfaces that have pin-holes and other irregularities. Their objective is to get a full thick coverage on large surfaces while knowing that it will be fully sanded before any further priming or topcoats.

Heavy Coats

Just after I started doing a topcoat, a thunderstorm rolled in and since the humidity was already pretty high, I was worried about more rain. The fact that the wind increased significantly wasn't helping things either and I'd already pushed the primer to topcoat interval to the minimum. A couple of the pieces could be layed flat so I chose to do them last and rather than a couple of light coats, they were given a fairly heavy coat; i.e. more than just a wet coat. These piece had some problems with solvent popping. The lesson learned was to be very careful about pushing the intercoat time and not to try laying on overly heavy coats. I'll leave it as a challenge to those that see my finished craft to try figure out which parts I'm referring to.

Light / Wet Coats

Since I hate runs, on the first topcoat I've tried to put on one uniform light coat (mist plus) and then go back and put on a wet coat about five minutes later. For the smaller parts, this has worked quite well. However, on larger parts using the detail gun in warmer weather I've found that the light coat has too much time to "flash" before the wet coat and I don't seem to get nearly as smooth a wet coat and I'm starting to get some dry spray. I think the solution is to switch to something like the slow thinner and/or to add a bit of retarder. Considering temperatures and the primer to topcoat delay, I think I'm going to start priming late in the evening and then apply topcoats first thing in the morning. The use of retarder has definitely helped a lot and even in cooler weather it helps to give more of the "plastic" look to the paint.
Update: I've changed my approach to this. I now put on a first coat that is pretty well a wet coat or just slightly lighter. I then go back about 1/2 hour later and put on a full final wet coat. This allows me to get a good depth of paint on the part and so far the only runs I've had are on very odd-shaped parts or when I wasn't paying a lot of attention.

Touch Up

I've painted a few pieces that I knew would require a bit of touch up due to the hanging wires or just that I've found a bit of hangar rash on other pieces. Since it's wasteful (and time consuming) to repeatedly mix small batches of epoxy urethane, I tend to identify these areas and then wait till the next time I'm spraying that colour. At the end of a spray session, there is usually a bit of leftover mixed paint that can be used for this. If these areas are on pieces that have just been sprayed and can't be touched for a day, I preserve a bit of the leftover paint in the freezer. While I won't use this preserved paint for spraying (it gets a bit thicker), it's perfect for brush touch ups. I've found the paint is useable after several days in the freezer, but probably not after a week or more.

Spray Gun Cleaning

I've been very conscious to carefully clean my spray gun at the end of each session, both externally and by running MEK through it. However, I haven't always been taking it fully apart and cleaning the needle. I was experiencing some trouble laying down a full wet coat and couldn't get as full a coverage as I was previously obtaining so I decided to do a full cleaning. Despite the previous cleanings and running MEK through it, I was amazed at the internal accumulation of paint. At no time had I ever left paint in the cup that could have gotten to the gel stage. Needless to say, I'm now cleaning the needle area much more frequently.
Update: After carefully checking during various spray sessions, I've come to realize that this "clogging" is mostly due to the primer. I'm aware that there are larger orifices available which help with thicker paints and if I was doing a lot of spraying I'd set up a special gun just for the thicker primer. This seems to be compounded in warmer weather where the primer seems to "kick" much quicker.

Ambient Temperature

The epoxy urethane paint that I'm using specifies a minimum / maximum ambient temperature of 15°-32°C (60°-90°F) during application but I've pushed the limits in both directions. Too cool and I have to add extra thinner to get a good spray pattern ... one has to be more careful of runs since they accumulate and form with a sort of delayed reaction. Afterwards, I move the part into the basement as soon as reasonable in order to let the paint cure at a more normal temperature. The high end of the temperature range causes a lot more problems since the paint tends to flash much quicker and is more prone to dry spray and/or orange peel. I'm always adding a bit of retarder now (even in cool temperatures) since it gives a smoother result and this is even more important with warmer temperatures. The other unique thing I've seen in high temperatures (over 25°C) is solvent popping even if I've only added minimal thinner. For large areas such as the cabin which are very noticeable, I'd recommend trying to apply the paint in the lower to mid range of the recommended temperatures.

Speeding up the Cure Time

One of the potential problems with the paints I've used is they have a long cure time from paint to handling and also to the full cure. I've accepted this for the main pieces and don't want to tamper with a known-good end product. However, there are a few smaller pieces in sub-assemblies where it would be a real pain to wait up to a week before proceeding. There are a few smaller pieces that I painted with Krylon Epoxy that I chose to oven bake at the lowest setting (~200° F) for one hour and this seemed to work extremely well.

Runs

I don't think any of us like to admit it, but runs can and do happen when painting odd-shaped pieces. If this occurs on the primer coat then it should be removed before the topcoat since the topcoat won't hide the run (it may actually magnify it) and the glob of primer is much softer than the topcoat or the substrate. If a run occurs on the topcoat then one has to make a decision whether it is in a noticible place or a relatively hidden area and can just be ignored. It is possible to try sand and polish a run, but this can make it even more noticeable since it's nearly impossible to match the gloss effect. Generally, one has to sand out the run then roughen the entire piece and re-paint it.

Filler

I had a few places where I wanted to clean up a few riveting oopsies and other blemishes. For the bigger areas I'd used Araldite and it works extremely well. On the very small areas I'd tried some "Spot and Glazing Putty". While quick to dry and very easy to sand, I can't recommend this product for this application as it seems to remain porous (for lack of a better explanation) even after a coat of primer. This was confirmed with a professional painter who recommended using one of the lightweight catalysed fillers instead.

Steel Preparation

All the tubing I've received either as part of the kit or extra purchases has a mill scale layer that needs to be removed prior to painting. Maroon Scotch-Brite works well for this, especially if used with some kind of MetalPrep solution, but it takes a lot of elbow grease. For the straight control rods, I tried chucking them in a slow turning lathe where about half the rod stuck out the back side of the power head. Hand applying Scotch-Brite to the rods while they were turning took the mill scale off in no time. The rods were then flipped and the other half of them subjected to the same treatment.

Painting Screws Heads

I've seen various craft where someone has tried to paint screw heads with less than stellar results ... either heavily chipped or gobs of paint. I tried this on a few of the stainless screws and seemed to get pretty good results. My technique was to first give the screw heads a light cleaning using maroon ScotchBrite followed by a solvent cleanup. The screws were then placed in a piece of cardboard that had holes punched in it, but also leaving a bit of thread showing. I left plenty of space around each one, perhaps an inch or so. They were then hit with a light primer coat and a wet topcoat. By doing light coats at all different angles, they seemed to get a uniform coating right up to and around the edges.

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Last updated: February 08, 2009