MOST wetsanding is done PRIOR to a paint job, on a primered and filled car. You have done all the rough sanding, all the body repair, and added a second coat of primer, and then used a guide coat for finish sanding. THEN, a final primer coat is applied and the entire car is wet-sanded with ultra fine sandpaper or emory cloth. Once this is done, the surface should be free from any imperfections and the paint should be perfect as applied, provided the guy spraying it has a clue and 20 years experience. Usually only the cream of the crop gets the wetsanded treatment, but it used to be a common practice for most street rods and classic restorations. Prevents a lot of the post-paint work of buffing, sanding, buffing, etc.....
REB