Fifty,
I do a good amount of plastic and urethane bumper painting on car parts. The process I use is to wipe down with SEM plastic prep cleaner, then use SEM "Scuff-n-Clean" with a red scotch brite pad, scour the plastic with the scuf pad dipped in the Scuff-n Clean compound. Wash with soap and water, dry and wipe again with plastic prep cleaner, then spray with epoxy 2K sealer, dry 1hr. min. then paint. This works well to clean the oils and mold release agents out of the plastic. They also make special primers and adhesion promoters for different types of plastics, but you really don't need them these days with the new cleaners and scuffing products available for use on problem plastics. If you use the epoxy sealer, be sure to read the tech sheet. Some brands like PPG's DPLF line will only adhere well to plastic if you use the slower activator. And only put on 1 medium coat, then topcoat it.
Like Swiss said, SEM makes some excellent products, you should be able to get their stuff from any good automotive paint supply house.
Ray