After reading my post I should clarify myself. The fillers I prefer recommend (z grip and rage) were not being recommended to repair the rust. Those are for finishing after the rust has been cut out and repaired with good metal. Filler is not a bad thing and will not cause any problems if the surface is prepared correctly and clean and if the filler is not too thick. I keep filler to a maximum thickness of 1/8". If it needs to be thicker than that you need to get the hammers and dollies out.