Hi Steve,
Well, you want to use a non-loading sandpaper like Klingspor's Aluminum Oxide open coat stuff. I've also used 3M "Sandblaster" paper that is a garnet based open coat paper. But I like the Klingspor stuff the best. I sand with this stuff then use my finger to "flick" it and the dust just drops off. I use either 280 initially or 320 depending on how heavy the primer is. Hit it with the 280 (if the primer is thick), but don't go all the way through the primer. Just get it leveled and the bulk of the material off. Then change to 320 to get down to the final surface. If you are really trying to get it smooth, you can then go to 400 or 600.