So the question is, what is the proper way to prep the plastic cover, fender skirts and other moulded plastic pieces for paint?
I use an oxidation remover like Jasco Prepaint Conditioner and a medium 3M synthetic scrub pad. After the scrubbing, a rinse. Depending on what type of plastic, I'll use a plastic adhesion enhancer and/or a good primer. That is followed by the finish coats. A final finish coat of high solids clearcoat is going to keep your paint on the plastic. The coats of paint are thin so that any abrasion could mean a scratch totally through the paint. The clearcoat helps to avoid that.