I painted my plastic. So far so good. I put clear coat on mine also. If I get a scratch, I just wet sand it with 2000 grit sand paper. I used a red scuff pad and comet (Yes the cleaner you use to clean your tub's and shower's) Scuff it all up really good. Rinse it all off and then do it again, and rinse. Biggest thing to remember is like thomasj said. Make sure it is clean, clean, clean. Then we used some dupont cleaner and degreaser prep rag's we had at work. They make two kind's. One for plastic, and one that will change plastic color. (I think, never used it on plastic so not sure what it will do) I dont think it will matter as you are going to paint it anyway's though. We used the one for plastic. As you wipe with these rag's, do not let it drie on the plastic, or it will fisheye when you paint it. Wipe it clean a few time's. You want it as clean and grease, oil free as possible. Once you are done, get a tack cloth and wipe it all off. This get's all the little fiber's and lint off from the prep rag's. Blow it all off and go over it again with a clean tack cloth. Make sure your shop, or whereever you are going to paint this at is clean, or when you blow it all off, and then paint it, you will have trash all in your paint, and it will look like crap. Then we sprayed it all with a plastic Adhesion Promoter. This is what I used and where I got it from.
Klean Strip Bulldog Aerosol Paint Adhesion Promoter GAL - eBay (item 280619280514 end time Feb-20-11 07:21:52 PST)
We put two coat's of this on and waited about a hour. This act's as a primer also, so no need to buy primer. Went back over it with another tack cloth. We then sprayed 2 coat's of dupont green metallic 5000 series paint on it with 25 gram's of extra metallic out of a gallon I had that I never used for a project. Let it drie, and clear coated it. We put 4 meduim coat's of dupont premier clear on it. All in all it probably would have been cheaper to just buy some good plastic off ebay or something if I had to but the paint and clear, but I got it for free. I think I have about 75 buck's into mine. It look's awesome, exspecially with all the metallic in it. I am not sure how much a quart of paint from them is, but I know I got a gallon of blue in the 6000 series's, and our dupont rep said it was $700 a gallon. A little over priced I think. If you can get everything for a decent price I would say go for it. I have no regret's with mine. If it ever start's to look bad, I will just sand down the clear, and reclear it again as long as the scratches are not to bad. I know you can get a gallon of flat black all over the net for around 60 a gallon so it should be a little cheaper as you will only need a quart as long as it is unreduced. If I can find my cord for my camera I will snap some pic's of mine and post them on here. I painted mine almost a year ago, and it still look's good. Most of the trail's I ride are cleared out pretty good, so that help's a little too. Hope this help's. If you got any question's let me know.