I use SEM Sand Free to prep plastic. I also bake them, gone both ways and it definately lasts longer. Don't sand too much because then your back into the release agents and you have to start over, not to mention you lose some of the texture. If your using true automotive paint, which is what I recommend, it really needs to be primed. Just be sure to use a flex agent in the paint or it will start to crack in the summer. The interior on one of my cars is almost 3 years old now with next to no chips or flakes. If you spend enough time in prepping it the finish will last as long as any other painted parts.
Ryan