USE THE DOG BEFORE THE COLOR COAT!
If painting plastic parts, buy a can of Bulldog Adhesion promoter (or the likes) to get the plastic ready for the color coat. The mold release oils in all plastic parts like to repel paint, and scratch easily or peel off in time.
Going cheap isn't the best choice with plastic painting. Cleaning off a "bad job" is a real pain in the arss!
Richard