I have a lit of rust that has formed under my gas access door for a second time now. The auto body shop said the metal is too far gone to sand it down and repaint it. What can be done to fix this area and keep it from rusting yet again? I live in northern Michigan, where the roads are salted, so it's very rough on the vehicles. One other body shop quoted $400 to fix it, as they said they needed to blend the paint!!! Ouch!!!