I would use 800 grit and you are just needing to scuff the surface initially. You sand after priming to insure a smooth surface free of imperfections. That is whem you would add any needed body filler to make the part perfect. That is the modification they usually refer to in the add discription. Any sandpaper with a grit # under 800 would be too aggressive and would likely leave scratches that would show through the paint. I have seen people use green srubbing pads for the initial scuffing prior to primer. Here is a link to common problems and solutions to them. Paint Problems & Solutions*::*BMC FISH EYES
Small crater-like openings in the finish
1. Improper surface cleaning or preparation, usually involving deposits of silicone or wax on the surface of the old topcoat.
2. Effects of an old finish with embedded silicone that is not removable by
3. Oil contamination of air lines.
1. Remove all traces of silicone or wax by thoroughly cleaning with a product designed for this purpose.
2. Add fish eye eliminator to the topcoat.
3. Drain and clean the air pressure regulator daily.
After the affected coat has set up, apply a double coat of color containing the recommended amount of fish eye eliminator. In severe cases, the affected area should be sanded down and refinished.