what i did for my body kit witch was use was sand it down with an orbital sander. Then ~200 grit to get all the deep stuff out. And then i went over everything with filler primer. Filled everything really nicely. Then after 2 coats of filler primer i went and wet sanded everything with 400 grit paper.
celco sounds right...400 is ok, it leaves a good scratch, I myself prefer to go with 600 grit but it wouldn't make that much difference anyway. What type of surface are you prepping? Metal, fiberglass or urethane? It can make a difference in the steps you have to take. Later
I'm with Lox...I would suggest 600 also. If you go too coarse, you'll leave some deep scratches that may be noticeable when it's painted. You just need to rough up the surface so the paint has something to adhere to.
I use DuPont 222S (adhesion promotor) prior to laying down the paint. I've used Bulldog adhesive promotor in the past too and it works well.