Last time I did it, it took fire AND the wire. If you use any of these methods, just go easy or you'll warp the metal. Like Bakaruda said, depending on how much is on there, sometimes new sheetmetal is the best way to go.
Bakaruda is right, but first find out why there is so much Bondo. My 67 Barracuda had so much more than it needed. We were also able to shrink the metal back some. I'm going to pay someone to finish it, though.