I cut a one inch square piece of old fender and use JB Weld to epoxy it to the inside of the fender. Fender and patch are clean and bright and degreased with laquer thinner. When that dries, JB Weld the hole, let it dry, then sand it flat and paint. Welding or soldering is probably stronger, but this method has worked for me over the years with no failures yet. Gary