Report SANDING ? in General Modeling Posted February 28, 2017 If the primer has a rough sandpaper-like finish, then yes, you should sand it. I'd start with 400 grit and gradually work your way up (600-grit, 800-grit, etc.) until the surface is completely smooth, then finish off with a rubdown with a micro cloth. For best results, when you change grits, sand at right angles to the previous effort.