I've always clean the metal down as much as possible before using any filler. I have used filler to hide a few small pin holes but these were on a beater driver. I also have cut out rot and used sheet metal and liquid nails to hold the new metal inplace, the smoothed it over w/ filler. Again not a show car, just a racer that didn't have to look that great. If this is nace car and you want to keeep it, do it right and see a body man.