800 to600 or 400 has alot to do if your sealing up the prime or painting on the primer with no sealer some of the sanding paper seams that it is abit coarser .on every day work at the shop i was at was 320 d.a then using the 3m gray back wet dry paper 400to600 the jobs that i wanted dead flat base i tinted the last coat of primer and sanded it out to 600 and shot base right on the primer