All good suggestions, but many are lost on me. If you are taking the body to bare metal, then sanding scratches are a moot point. Certainly no one will be painting on top of the first coat of any primer. The first primer coat will be an initial protective coat. Then you have to find your high and low spots and fix those. Then more high build primer and careful block sanding and so on. So, scratches in the metal will be long gone short of using a grinder.
