I know Krylon makes a spray paint called Fusion that's designed to molecularly bond with plastic; I'm not sure if they make something similar that can be applied with a brush. You might also check on the paints that they sell in craft stores near the model car kits, since they're obviously designed to be used on non-porous surfaces, but I'd ask somebody at the store before using either just to make sure that the paint won't damage the surface that you're painting.
