I have a dilemma with some pitting I discovered on the roof. I started stripping it and sanded to metal and found that filler had infiltrated the pits to where I can't get it out without fear of sanding the metal too thin. The car from the fender tag had a white vinyl roof on it and I guess it percolated some rust under there. Is there a chemical to remove the rust other than a POR 15 remedy? Any thoughts?