Glidden Gripper primer. Its the best for painting plastic. I used it on a plastic set of outdoor house shutters and its still holding up after 3 years. Its so good you can actually paint ceramic tiles after its been primed with Gripper.
Get a DA sander with a soft foam pad and 80 grit sand paper. Sand that #### down. Then spray with a 2k build up primer surfacer. block with 600 to 800 grit paper smooth and flat. Shoot with a primer sealer then base coat and clear. This is the same process that is used on your truck, and to tell ya the truth those adhesion promoters are garbage. The automotive process is flexible too if you use dupont or ppg paints.