The best thing to use on plastics is autobody paint.
the best way I know how to do it is:
Wet sand 500-600 grit.
Get an adhesion promotor like bulldog or duplicolor adhesion promotor.(can find at autozone.)
Then use autobody paint (find at most auto stores)
Then a autobody ployurathan(sp?) clear coat (find at most auto stores)
If your on a budget I've had realy good luck with Krylon Industrial Osha spray paint. Its not glossy but its extremely strong, I've ran screw drivers, razor blades, bent the hell out of them, and never chipped or scratched it. the only way I can take it off is with paint stripper.