[MAStuart]

First off 800 /1000 grit is way to fine for primer to get a good bite to. Most places would be using 80 to 180 to prep the area for primer. The primer would then be sanded with whatever the paint manufacture calls for. A lot of base coat call for 400 grit but some calls for as fine as 800 grit. Most of the time bass coat is not sanded unless it is screwed up . Then it is usually resprayed then clear coated. If the paint is to be blended into the area around the primed area it needs to be prepped for blending. If the area has hard lines to be painted to after the area has been primed the whole area will be sanded with whatever grit the basecoat calls for.