http://www.4strokes.com/tech/plastic_restore/
Plastic Restoration via Painting by Edward Pinzel
It is no secret that dirt bike plastic fades and gas tanks turn yellow as well as lose decals as they age. Decals falling off and yellowing of gas tanks is believed to happen as a result of fuel vapors escaping through the pores of the plastic. After some good advice from others, this is what I have found that works well for me. I have done about five tanks and lots of plastic, and they all came out beautiful.
Although the directions below are geared more toward a plastic fuel tank, they are basically the same for plastic fenders and number plates (minus the first step and any internal prep and treatment).
1. Remove Petcock, tank mounting hardware, and gas cap.
2. Wash tank with a good cleaner, like Castrol Super-Clean, both inside and out. I use a skowering pad (like Scotch-Brite brand) to really rough up the pores, using the cleaner and rinse out well with water. It is advisable to use rubber gloves, as this cleaner sure can dry out your hands. If there is varnished old fuel in the tank, an automotive grade acrylic lacquer thinner or Acetone should be pored inside the tank to loosen up the deposits. I shake the tank vigorously then empty the solvent into a plastic container.
3. Now wash the outside of the tank with acrylic lacquer thinner or Acetone. I use a parts brush in a large plastic container. What was left over from the above procedure should work fine. This is to remove most of the old decals and adhesives, as well as any remaining grease or grime in the plastic pores.
4. The tank should now be flushed with soapy water, both inside and out, and repeatedly rinsed with clean water.
5. It is now time to sand the tank. I use an orbital auto body sander (dual action or DA sander) and start with 80 grit on the deep scratches and gouges, if there are any, as well as to remove old decals. I then take 180 or 220 grit on the sander and do the rest of the tank, as well as the areas that were sanded with the 80 grit, in an effort to minimize the scratches. Try to keep the sander flat at all times.
6. A skowering pad is then used on all the edges, the undersides, and any areas missed by the sander, to promote adhesion for the top coating process.
7. The tank should now be blown off with compressed air. Then washed again, both inside and out, with soap and water and thoroughly rinsed. Allow to air dry completely.
8. Wipe the tank down with automotive grade wax and grease remover and use a tack cloth (automotive grade) to remove dust particles. The tank should now be ready for top coating.
9. A product that I highly recommend for promoting good adhesion to plastic parts and personally know works very well is Sherwin Williams # UPO7227 Adhesion Promoter. This is available in aerosol spray cans as well as quart cans. The adhesion promoter should now be applied. I apply 1-2 wet coats, and let dry for about 30 minutes. I was able to do 4 tanks with one 16 oz. aerosol can.
10. You are now ready for the priming process. I used a flexible primer manufactured by SEM. It is available in quarts and possibly aerosol. The SEM part number is 39134 and it is called Flexible Primer/Surfacer. This is a great product and builds nicely. It can be sanded after 45 minutes. I apply at least 2 heavy coats, let dry, then wet-sand using 220 grit wet/dry sandpaper.
11. The tank should now be thoroughly rinsed and dried. Apply another 1-2 good coats of SEM # 39134 Flex-Primer. Let it setup (dry) then wet sand using 400 grit sandpaper. Thoroughly rinse again, and assess your work. It is possible that the tank may need another application. If so, repeat as necessary, final sanding with 400 wet. When finished, rinse the tank again thoroughly, both inside and out, and let dry. On my original Can-Am MX-3 fenders (red fenders in photo below), I had to perform the priming/filling process 3 times to remove all the scratches, as one of the fenders was really bad to begin with.
12. You are now ready for top coating. Remove any oils from handling using wax and grease remover. Use sparingly as to not saturate the current layers you have finished thus far. Make sure the tank is completely dry. Now wipe using a tack cloth to remove dust.
13. Any places on the tank where bare plastic is showing, where the SEM primer has been sanded through, should be covered lightly with the UPO7227 Adhesion Promoter. There is no need to cover the entire tank again with the adhesion promoter but it wont hurt if you do.
14. Allow to dry. Tack dust again using tack cloth. Now the color is to be applied.
15. I have always had good success using Sherwin Williams automotive finishes. I use Sherwin Williams acrylic enamel with hardener for most plastic. Now select the paint color of your choice.
16. The Sherwin Williams acrylic enamel should be mixed as stated on the can, using 8 parts color, 4 parts reducer, and 1 part hardener or catalyst. The following additive should also be added: Sherwin Williams Multi-Flex, Flex Additive, # V6-V299. Two parts should be added to the mixed paint. Ensure you are using an OSHA approved paint mask, as the mixed paint contains volatile chemicals, including Poly-Isocyanides.
17. I generally apply 3-4 good coats to ensure full coverage, obviously allowing the paint to flash between coats to minimize sags or runs.
18. The paint should be allowed to dry completely. I generally wait 2-4 weeks, then apply the decals. The finished tank can be washed and waxed, and cared for like any automotive finish, but waxing should be done only after about 4 weeks time, so any remaining paint vapors are allowed to escape.
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We asked Edward how the gas tanks were holding up after a long while and here was his reply:
"The finish holds up incredibly well, as it is an automotive paint re-finishing system. There are sealers to seal the insides of these tanks, and they will work well for preventing the gas fumes from trying to escape through the plastic pores of the tank. When the fumes try to escape, from leaving the gas in the tank for prolonged periods of time, there is a bubbling problem that is created near the bottom of the tank. I have done many tanks in this manner, and the ones that have a good tank vent, and are drained and flushed with water after riding seem to last the longest. Those that leave gas in the tank without sealing the inside seem to bubble near the bottom. Those that seal the inside of the tank seem to last the best, and the finish on the tank is as durable as any finish on any automobile, as automotive refinish products were used to restore the plastic above."
