Once you feel the area is back in shape enough to start filling with body filler, take the time to remove all paint and/or primer from the surface. Body filler needs to be applied to a clean and scratched surface. Since we aren't trying to grind a ton of metal off the panel, we use a disk of tired, worn-out 50-grit to take the primer off. Holding the grinder as flat to the surface of the panel as possible will prevent grinding too deep and will take a slight amount of metal off of any high spots that may have been missed with the hammer and dolly. Just don't forget how thin sheetmetal is by grinding right through it. We really only want to take just the primer off, leaving a clean, scratchy surface for the filler to bite into.