PREPARATION and PAINT
N.B. When I talk about zinc chromate primer, I've mostly used Endura EP-2C Primer Sealer. This is actually a two part epoxy urethane primer that goes on as a light-medium green and has a bit of zinc chromate added to it. After it has fully cured, it has a lot of the same properties as the topcoat. I do have aerosol cans of the more normal zinc chromate primer, and although they are handy I seldom use it. One of the problems is that even after a full cure, if one goes near it with MEK, acetone or other powerful solvents then it will be wiped off whereas the epoxy urethane primer is much more solvent resistant after it's full cure.
PAINT
Initially I was planning to have the frame powder coated. After spending a lot of time reading about the various techniques and products, I eventually came to the conclusion that I would not be comfortable with this after hearing how the coating can flex and mask the initial signs of weld cracking. I think it's also noteworthy that the NHRA has banned the use of powder coating on dragster frames for this exact reason. Another negative of powders is that it can be very hard to do touch ups afterwards and one really wants to be sure that they have all the tabs and other welded pieces in place before going this route. I understand that there are some new powders that go on very thin, but I have no experience with them and would first want to make sure that the coater is very familiar with them. The use of a light colour on the frame also gives an additional visual feedback of any cracking as it will show up as a dark line after cleaning.
As far as paint types and brands, this also caused me to do some research. Based on past experience, I am very impressed with the epoxy type paints both for their gloss retention and overall durability. However, I have also seen poor applications of this by inexperienced painters who aren't familiar with the right amount of catalyst and retarder to use given the conditions. I'm just starting to look around for a professional shop that is experienced with the application of these paints. As far as brands go, I will probably stick with the "Canadian" philosophy and use Endura. Although I haven't worked with it for many years, I've been very happy with the results (appearance, gloss, hardness, durability etc.) when used on models subjected to high vibrations and concentrations of nitromethane (a paint solvent). Also, one of the first times I saw this paint was on a fleet of cement trucks that were always sparkling even though they're washed down with muriatic acid.
One of the other finishing processes that I have seriously considered are the products from Aircraft Finishing Systems. The real appeal to me of these products is the fact that they have low Volatile Organic Compound content and use water cleanup. For those that aren't familiar with the hazards, I would STRONGLY recommend that they do some research before using any of the epoxy or polyurethane products as many of these are extremely hazardous to ones health and require special procedures and protection for both their application and disposal.
I've decided to go with the Endura products for painting as they're readily available and used often around here for aircraft painting, especially commercial fleets. Of course the hardest part is selecting the colour scheme and actual colours. I wanted to use a VERY bright red without going too much into the orange tones ... sounds easy until one finally tries to get the paint mixed. I had managed to get the colour I wanted out of a spray can and made a few test strips to verify it. However, the zinc-chromate primer on the frame made it turn out much darker than I wanted and it turned out that I had to put down a light coat of white as a base before applying the topcoat. There's also an irony in the choice of the red colour that I selected. Red is a very difficult colour to get a VERY bright shade without going into the orange tones and I had the option of going with a custom mix or a mix that was on file. I eventually went with a 1994 version of Air Canada red. Those that know me well know that this is my carrier of last choice so perhaps it is ironic that I'm using their colour to create an alternate mode of transportation.
The Construction Manual talks about a C.H.R.I. optional painting method of just preparing the metal and then applying topcoat without a primer. I have seen the results of this and I'm not impressed. Perhaps it was the painter (actually there was dry spray, orange peel, thinly coated areas and other issues). The bigger issue I saw was that there were already areas where rust was developing. I also noticed a few hangar rash and/or rock nicks that were also developing rust. Perhaps it was from a poor topcoat application but I feel that the primer adds a lot of protection to the underlying structure and provides better adhesion. If the primer and topcoat are different colours then it also makes it much easier to see if areas have not received a full topcoat.
One thing I wanted to do was to have my tanks (fuel & oil) in a very reflective colour. Originally I'd thought about just highly polishing them but that would require some continuous maintenance unless they were clear coated. As my paint scheme evolved, I decided that I'd rather have a gold colour rather than a silver or chrome colour. The ideal colour would be what can best be described as "gold chrome" ... those that have seen it can understand the reference. Since I knew this would need to be contracted out, I did some searching for it and it seemed like the best alternative was a special powder coat. However, the local powder coater warned me that it is very hard to get a consistent colour and even went so far as to do a sample for me using the brass version which they had on hand ... I was disappointed as it is essentially a translucent tinted coating that collects in any corners and edges which then deepens the colour in these areas. Since I wanted to get this over and done with, I finally went with a standard gold powder coat and a clear UV topcoat with a fine metalflake.
Long after I'd had the tanks coated, I finally ran across a couple of products that might have given me the results I wanted. These seem to be relatively new processes and appear to be in an evolving field. Specifically, if I were to investigate this again then I'd seriously look at trying to find someone that works with the Cosmichrome and Spectra Chrome processes. I'm sure that a bit more research would also turn up some other products out there.
Aluminum Preparation
For this discussion, I'm assuming that the aluminum parts have been properly fabricated and that all holes and edges have been properly deburred and smoothed. There were several areas on my pre-assembled cabin that had very obvious tooling marks. The cheek sheets and belly pan had marks from a metal shrinker and the door frames also had deep gouges from the forming. While I had thought about using a bondo-type material, I used Araldite 1258 to fill and smooth these areas. This is an aluminum-filled aerospace structural repair adhesive / filler that is capable of being sanded and machined ... sort of like a high tech JB-Weld for the aerospace industry.
Originally, I wasn't sure how much preparation I would be doing on all the aluminum parts as this is somewhat dependant upon the final primer and paint selection. I have both Alumiprep 33 and Alodine 1201 which I had been using on various smaller parts. I've also found a good source for doing anodizing and will use this as appropriate.
I decided to alodine all of my parts before riveting them together. I've found that the majority of my pre-paint preparation time is not the alodine process, but rather the process of cleaning and preparing the parts, many of which were fabricated quite a while ago and have been exposed to quite a bit of dust and other contaminants. After an initial hit with the air nozzle and a quick wipedown with solvent, the next step is to double check that all edges and holes have been properly deburred. I'm then using maroon Scotch-bright pads lightly hand-rubbed to actually clean the surface. While this does produces a good surface for the paint, it also produces a very fine dust that requires a lot of passes with a solvent-soaked throw-away rag. I use small swatches of shop towel (1/4 sheet which is then folded) dipped in MEK or acetone for this purpose and it works well. After all the time doing this cleanup, another 10 minutes for a corrosion-resistant coating seems like a cheap investment.
For the smaller parts, I obtained four large (but
shallow) covered plastic tubs into which I've put about an inch
or two of:
1) Alumiprep (1:3 with water)
2) Rinse water
3) Alodine (1:2 with water)
4) Rinse water
After a batch of parts has been cleaned, it's then a simple matter of putting them in the first tub, wait 3 minutes or so, rinse in tub 2, put them in the alodine tub for 3 - 4 minutes, rinse in tub 4, rinse under running water, and then hang them to dry. In about ten minutes total, this produces a batch of parts that have been etched, corrosion protected and are ready for primer/paint without any further actions. If I don't rinse the Alumiprep'd parts under running water, it is necessary to change the water in tub 2 quite often, depending on the size of the parts. Since the tubs are shallow and without racks, for flat pieces I just flip them over every 15 seconds or so in order to give both sides equal exposure to the Alumiprep / Alodine. GENTLY rocking the tubs also seems to enhance the effect.
I've had great results doing the immersion type of preparation and can get a good "gold-green tinged" result following the manufacturers recommendation for dilution and dip times. In fact, I even threw together a set of larger trays (~ 9" x 45" x 3") made out of acylic so I could do the ribs and a few other larger parts. That still left some large parts that I couldn't immerse and I had to resort to the wipe-down technique. Although this works, I find it much harder to get consistent results and it is imperative that the part remains wetted down during the process. Scotch-brite pads work well to apply the Alumiprep and I use a sponge to apply the alodine. If one can find a commercial shop that can immerse large pieces, then it sure would make it easier and would probably result in a more uniform coating.
One of the concerns I had with alodine is that many of the datasheets that I've looked at recommend that after alodining the parts they should be primed within 12-24 hours. This isn't always practical for me and I sought an opinion from someone who is very familiar with the various conversion processes and requirements for the aerospace industry. He confirmed what I believed, which is that the primary objective is to keep the surfaces contaminant free between the alodine and primer steps. I'm trying to only handle these pieces with gloves and to keep them physically protected from rubbing, dust, oils, etc. The other new information I received was that stainless steel should be primed within four hours of cleaning, such as with Scotch-bright and MEK, in order to prevent it from forming a chromium oxide film which will interfere with paint adhesion.
Note that once the parts have been prepped and alodined, they are truly ready for paint but sometimes this is not always practical. If the parts are not going to be immediately painted, I place them inside plastic bags once they're completely dry and this makes for easy handling. I found some cheap extra-large freezer bags and also the large clear garbage bags normally used for leaf collection work well for this purpose.
One debate I've had with myself is whether to prime various parts before riveting them together and thus sealing and protecting faying surfaces. This is the proper way to do it according to the various Mil-Specs which also state that either exterior exposed edges or faying surfaces of dissimilar metals should actually be done with sealant rather than primer. Even though I live in a relatively dry climate, as a minimum I will have both surfaces alodined before riveting and so far I'm using the home-builder's happy medium of joining the surfaces with a coat of wet primer between them. For those that want an introduction to faying surfaces treatment, there is a good article here.
Steel Preparation
I waffled about the alternatives for quite a while. The construction manual talks about just using Metalprep and Scotch-bright. I tried this and it does work, but it does take a LOT of elbow grease and because of the water cleanup it must be primed and/or painted immediately. I also considered sand or bead blasting but was very concerned about some of the horror stories I'd heard about actually ruining the frame from using too coarse a sand and/or being too agressive.
I ended up getting my frame and some of the other steel parts sand blasted. However, the deciding factor was that I found a shop that is very familiar with doing this on the frames of certified aircraft. They are aware of the requirements and pitfalls and only use the finest sand, very low pressure and only enough exposure to just remove the mill scale or rust. The other advantage is that they also have a paint booth and can prime the frame as soon as it has been blasted. In hindsight, I wish that I'd also considered having them lay down the base colour coats as their painter did an excellent job of laying down the primer on the very difficult to paint frame structure. However, I was concerned that I wanted to use multiple colours and that there is a 24 hour recoat window on the paint I'm using.
I always dread the final cleanup in the preparation for painting since it seems like no matter how much time I allow for this, it always takes longer. The frame was no exception since it had received many coats of WD-40 over several years and this needed to be removed prior to blasting / painting. I first tried getting this off with straight acetone, but it took many passes to remove all traces of the oil buildup. I then tried using a de-greaser first and that worked extremely well. One quick pass with the de-greaser, one pass with acetone to clean off the residue and a final acetone pass to check for any spots that were missed.
Tip: When doing the de-greasing and solvent process, ALWAYS wear a good pair of protective gloves. I'll admit that I did a little bit of this process one day without gloves and it really screwed up the skin on my fingers for several weeks ... sort of the same sandpaper-like effect as having cyanoacrylate glue on them.
For the smaller steel pieces, I went with the basic Metalprep (actually DuPont Metal Conditioner 5717S) and Scotch-brite process. This seemed to work quite well, but was time consuming and messy.
Nutplates
Nutplates are crimped so that the last couple of threads before the screw exits effectively create a clamping pressure rather than just a simple threaded nut. I believe that the F5000-08 floating nutplates that I received with my kit came from a batch that were not properly manufactured. Several of the nutplates on my pre-assembled cabin and shroud were extremely tight and a couple of them contained screws with stripped threads. Ironically, most of the other ones had absolutely no clamping effect. Most of the supplied new nutplates only allow a screw to be turned about 1/8 - 1/4 turn when handheld before they become too tight for turning the screw with just finger pressure. I have a lot of different nutplates, including more of the F5000-08's, and none of them are this tight at the initial entry. I finally resorted to testing each nutplate before installation and for just about all of the supplied nutplates I had to run an 8-32 tap part way into them. After a bit of testing, I found how far my tap had to be threaded into them and this became a task that could be mindlessly done while watching TV.
As for preparing a piece to accept a nutplate, I believe the most important aspect is to carefully position the two rivet holes and then to properly countersink them. The construction manual shows the pictures from Tony Bingelis's "The Sportplane Builder". This technique works well, BUT ... it should be noted that the illustrations show a fixed nutplate vs. the floating nutplates that are supplied with the kit. Unless one is very careful about centering the floating nutplate when using this method, the rivet holes will not be centered with the screw/bolt hole. Many of the nutplates on my pre-assembled shroud were off center and some of the screw holes had even been ovalled to accomodate this. There are two other alternatives: use a nutplate drilling jig (about $35) or get some fixed nutplates for use as jigs during fabrication. Note that if you use the fixed nutplate method, unless you're extremely careful the rivet holes in the ears will eventually become ovalled after repeated use, especially if you use cobalt drills. Even the drilling jigs are prone to this and I found that mine is now barely useable after I used it to drill all the nutplate rivet holes in the steel frame tabs.
The .032" and thicker panels can be countersunk to accept the nutplate rivets, but .032" material is pushing the limits. In order to maintain the spacing and obtain accurate holes, I found that it was necessary to backup the sheet with a clamped block that had a #40 hole in it that would continue to guide the pilot on the countersink. On flat sheets, this was simply a hole in the top of my workbench. For the .025" material that I used in my floorpans, I elected to use nutplates that were pre-dimpled and instead of having to countersink this thinner material, it was a simple matter of using a 3/32" dimple set in my rivet squeezer. I really like these dimpled nutplates as they are self-centering and the rivets are always flush. Most of the nutplates I've installed are using solid rivets and I much prefer to install them with the hand squeezer and flat dies rather than use the rivet gun. In a very few places I've had to resort to using pulled rivets since there is no rear access for either the squeezer or a bucking bar.
I have seen various discussions on whether nutplates should be primed or not. Although there was no clear consensus, I'm priming them since I have seen nutplates with rust streaks, especially the floating plate style. First I take a batch of nutplates and give them a good bath in MEK, then spray the bottom of them with a light coat of zinc chromate. The panel is alodined, as appropriate, and I then use a small brush to give a light coat of primer just to the area of the panel where the nutplates rest. The nutplate is then cleco'd or clamped in postion for a minute or two and finally riveted in place while the primer is still wet. The entire panel is then primed, including the topside of the nutplates, and painted as appropriate. So far, I've had no problems with floating nutplates sticking after the primer but I do try to move the floating part around within a day or so after painting with a topcoat. I'm also putting a drop of Corrosion-X on the floating nutplates after everything is painted and just before final installation ... I HATE the sight of rust.
In Hindsight
After all the mixing of various epoxies for primer & topcoat, intercoat times, etc., the joking side of me says that I should have just gotten a couple of cases of Rustoleum spray cans, or something similar, and just gone to it. No mixing, no gun cleanup, no curing delays etc. Considering the price of high quality epoxy urethanes, there probably wouldn't be any difference and in fact probably a considerable saving.
On a more serious note, if I was starting over then I'd probably give a lot more serious thought to just using standard automotive finishes. The downside is that they're not nearly as chemical proof and I don't believe they're as "tough"; ironically this is an advantage insofar as showing any frame cracks. On the positive side, there's an unbelievable choice, they're much faster drying / curing and any paint shop should be able to assist with their application (for a price of course). Given that one is familiar with applying them, it's much easier to get the super gloss "show special" finish, especially with a clear topcoat. Another negative is that one would need to be more careful with the actual surface preparation.
One of the subtle advantages of the epoxy urethane that I used is that the surface does not have to be absolutely perfect. By the time one has applied the primer and at least one wet topcoat, small imperfections will disappear. However, that isn't to say that one can use course sandpaper or leave major scratches in place ... in fact, one probably wants to remove these flaws more from a structural standpoint than from a paint preparation standpoint. I know that I probably spent too much time and care in keeping the aluminum parts in good shape and then going through the final surface preparation.
CONTRACT PAINTERS
One of the problems of using a contract painter is that one has to prepare a large batch of parts to be painted at one time to make it worthwhile and this is getting to be a real pain. I'm a lousy painter and while I might spray some primer and small or hidden pieces, the frame is definitely a challenge to get a good topcoat on. The other issue is that since I used multiple colours on the frame, I had to be present to show where the colour changes occur. Even then, there was some mis-communication with my painter and there are a few areas that got an extra coat of colour that wasn't required ... since this is on the hidden cabin tubes, it doesn't matter cosmetically but it does add a bit of unnecessary weight. As an owner / pilot, I also think we are much more conscious of the weight buildup whereas the painter I used was more worried about coverage.
If I were painting the frame again, there are a couple of
things that I would do differently. Unless I had access to a
LARGE paint booth, the main frame and the boom would be painted
separately since there is a lot of overspray when working around
tube structures and it is necessary to continually look at the
tubes from all different angles. I am now aware of several
builders who chose to make rotating jigs to aid in painting their
booms. While the painting can be done in a smaller booth, it
becomes crowded and awkward which makes it very hard to lay down
a good final topcoat. On my main frame section, I wanted a red
topcoat behind the firewall and we left the area forward of this
white. Because of limited booth time and paint-to-mask time, the
painter didn't mask the part of the frame that won't show ...
it's now an interesting shade of pink and red due to the heavy
overspray, but I'll probably choose to sand this off since I'm
sure it'll bother me every time I work under the seats.
Interesting how his definition of overspray is radically
different from mine.
Update: I started sanding off the overspray in the hidden cabin
area since there was an amazing amount of buildup ... some of the
countersunk holes for the 3/32" nutplate rivets were almost
filled! My intent was to sand this so there is just the thinnest
layer above the primer, without actually removing any of the
primer, and then put a THIN topcoat on to seal the
sanding and put a uniform colour back on. The moral of the story
is to take the time to do it right the first time as it takes a *LOT*
more effort to correct the mistakes afterwards.
Update2: Because of difficulty with access to a paint booth, the
amount of masking that would have been required and the
difficulty working around the mask, I ended up brushing the
topcoat on. Although this area is hidden during operation, the
results actually came out much better than I expected.
Ironically, it took a lot longer to brush these tubes than I
expected and it would appear that one has a choice of either
spending their time masking and spraying or brushing ... the
brushing probably took longer.
I'm now kind of hooped! I've decided that the orange peel and dry spray on the boom is bugging me so much that I really don't want to leave it that way. My eye is always drawn to all the mistakes and I just get more frustrated every time I look at it ... what a !#@^&* screw-up. That leaves me two options:
The dilemma then becomes what to coat the boom with after its been prepared again. I know that the urethane paints can do an excellant job, but it is very dependant upon their application. Obviously I chose the wrong painter the last time around and I'm paying dearly for it. The other option is to re-examine the powder coat options. Perhaps I will talk to my local coater again and see whether they use any of the newer thin films and what colours are available. Obviously this throws off my whole paint scheme as I know that I won't be able to match the colour that's currently on the main frame ... perhaps it needs to be redone as well.
I've since had a very interesting conversation with a member of the local experimental aircraft club who actually teaches aviation painting and finishing at the local vocational college. He is fully aware of the difficulties of painting a truss frame and offered up a few hints. Using a detail/touch-up gun will tend to reduce the overspray as will the use of a handheld piece of cardboard behind the tube to act as a shield. Obviously the paint has to be mixed properly and the use of reducers and slow thinners may help in achieving a smoother surface. The use of electrostatic equipment would also be of great help, but it's not that common. A heavy duty gun is exactly that and will probably lead to a lot of overspray and a heavy finish.
I also got an email from another Safari builder about this. He said that he'd received a tip to try connecting a 12-volt battery to the frame and the gun to create a form of an electrostatic system. Unfortunately he'd already painted his frame and couldn't give a first-hand account of whether this technique works.
I chose a different painter who does a lot of aviation work to do the exterior of my cab and I wasn't overly impressed with his work either. The reason I contracted this out is that it is a highly visible large area and I know that my relatively cheap HVLP spray gun tends to give me orange peel. Ironically, I believe it's only slightly worse than what this "professional" did and certainly my detail gun is a lot better. I'm totally sick of painting and figure it's just time to move ahead with final assembly.
Final thought: As the above shows, I contracted out the painting of the main frame, the boom and the exterior of the cabin. I was so disappointed with the results that I decided to do all the rest of the painting myself even though I don't consider myself a good painter. If I was to do this project again, I'd either carefully research and check out various painters well in advance or spend the money to get a good paint gun and rent booth time as appropriate.
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Last updated: February 08, 2009