Before the primer or before the color coat? I used 80 grit on the bare metal, primed with epoxy, filled minor dents with bondo, scuffed with 400, used spray polyester to block, finishing up with 400, then a couple of coats of primer-surfacer.
Then I tried something different. 3M makes a Hook-It II soft interface pad for your DA so that you can dry sand with 500 grit (or even 400, they say). Eliminates the mess of wet sanding and works really nice. And very easy. And the powder guide coat stuff they sell is also very nice, much better than spraying black paint. It looks like copy machine toner. I wouldn't be surprised, considering it's 3M.
I highly recommend a manual called "Modern Techniques of Auto Restoration" by Ken Wickham. This guy knows his stuff and explains everything clearly.
