I have a thread going about painting interior, haven't started the painting process yet BUT have been doing a ton of reading/research. Focus on the cleaning/prep stage more than anything. I have picked up many SEM products to ensure that the plastic is completely free of contaminants and ready for paint.
SEM paint
Adhesion promoter
SEM plastic prep
SEM scuff/clean
Gray ultra-fine scuff pads
I go overboard when I do projects, and I've already given the pastic two passes with some wax remover (front and back) before I even start the SEM cleaners and pastic prep. I'm using a flatner in my paint because of the look I'm trying for, as well as a cross-linker so any cleaning of my interior later on doesn't rub off my paint.
Hope this points you in the right direction. I'll be doing a howto video in my thread when the weather cools down a bit here.
B