Remove any metal that is not solid from around the area of the hole that is to be repaired. Use coarse-grit sandpaper to remove any paint and primer for a distance of 3 to 4 inches in all directions away from the hole. For the best and longest-lasting repair, all the rust should also be removed. It is likely that some small rust pits will remain, however. Use nylon window screen to fill in behind the hole and create a backer to hold the auto body filler. Make sure that none of the screening is protruding from the hole. The screen can be bunched up and pushed through the hole in areas where access from the rear is impossible.