When a metal tub chips, the black primer underneath makes the damage stand out. The best repair results come from hiring a professional repair service, but the cost can easily be a $100 or more. But there are some do-it-yourself options which can improve the appearance. If there is loose rust in the chipped area, scrape or grind it off. A small stone bit in a drill or sandpaper by hand works. Hardware stores sell color matched tub and appliance touch up enamels for several dollars. They come in small 1/4 - 1/2 ounce bottles with a brush applicator in the cap. See the picture below for examples. 

Note that if the tub is white, there are different shades. Most shades of white are matched to American Standard, Crane or Kohler. The enamel touch up is simply brushed on over the chip so that the chip void is slightly over filled. Note that if the chip is on a vertical area, the enamel will have to be layered in over time as laying it on thick will likely cause the enamel to run. Once the enamel has cured dry, sand smooth with 600 to 1000 grit sandpaper. A buff with gel gloss or any automotive wax will give it shine.

A quick cover up for white tubs can also be achieved by using the white off correction fluid typists use as a substitute for the enamel. It is water proof

With fiberglass units, do-it-yourself chip repairs can be done much the same way. Chips are static. I.E. there are no structural issues with them getting worse. Cracks however, tend to propagate. So this method is not effective for cracks. A professional repair is about the only option.

Another option for chips on the bottom is to hide them. Cover them with a stick-on anti slip decal or strip.