We do this all the time in our manuf. shop for preping and painting plastics.
Plastics have an oil in them that moves to the surface to help release from the mold, or they simply have a release agent on them. This oil is what prevents paint and adhesives from sticking. We have found that the easiest way to remove the oil is by flaming it.
Take a fan tip on a propane torch and run it over the surface so that the tip of the flame is just touching the surface. Move at a steady pace, not too fast, and not too slow. You can sometimes see the oils, it looks like they "disperse" as the flame goes over.
After you've flamed it, paint it. Just try not to add more oils before painting by touching the surface with your bare hands.