Majicmike is exactly right. That is the easiest and best way to do it. In fact at the custom body shop I work at, that is how we would do it. The problem w/ sanding the plastics so course is that by the time you are all said and done, the primer buildup is to thick and regular primer is quite brittle once completely dry. Urethane automotive primer is a little better, but you still run into a problem with being brittle. Since the plastic does flex when riding, the brittle primer is prone to cracking which in turn will crack the paint. Also sanding primer with anything more then 600-800 may actually be hurting your paint job. The paint needs those sanding scratches to adhere to the primer so if the surface the paint is being applied to is too smooth, the topcoat doesn't get the adhesion it needs. I typically sand w/ 400 or 600 before i apply my basecoat.