don't create more problems than you currently have, you're dealing with stretched metal, it doesn't have to do with work hardening, and over heating it will just blow holes .... sounds like that small section has been abused too much to simply hammer flat, any metal straightening needs to pretty directly "reverse" the direction of force that caused the damage, hard to do with creases .... if you can't fill it with weld and grind down both sides, you should cut out the damaged area and replace it, a thin skim of bondo is the last step to smooth an area, not fill damage
Bill
