this all depends on what you want to do. if you're just painting fillers, then i'd scuff them up with a red scotch-brite, spay some adhesion promoter on them, and then just paint them with some dupli-color or plasti-kote paint.
if you want to use a gun, then you need to pick a paint system and find what those require, which your paint store guy can help you with. some paints require a flex additive. and oem filler's are urathane (i think) which has many opinions on how to paint.
i use acrylaclean wax and grease remover, which is good to use no matter how you paint.
bulldog use to be the good adhesion promoter. might still be.