I just started stripping little areas of rust on my roof. i found that the roof has a pretty good size dent in it, once all the bondo had been grinded out I tried to knock the dent out more, but gives me a tin can effect. Can i just add heat to shrink the metal, then hammer and dolly out?

 

don't start shrinking yet. get all of your low spots out, as many as you can and get it as close to straight as your patience will allow before you use heat. using heat to shrink sheet metal should always be the last resort. reason being if you have an oil can and you still have a bunch of low spots chances are that the metal is'nt as stretched as you think and it is just the low spots keeping pressure on the high spot pushing it out. if you get it as straight as you possibly can and still have an oil can than just the torch to shrink it. heat the high spot that you wanna shrink right on the head of it until you have a cherry red circle about the size of a dime, tap it lightly a few times with your hammer to work it in the right direction and then take your blow gun and cool the spot. You will actually be able to watch the metal shrink back into place. Just remember that once you start this process you could be opening up a can of worms and you'll probrably end up chasing that high spot all over your roof until it finally works itself out. Just be patient that is my best advice.

 

If you use heat, you shouldnt need to hammer the metal while its hot, in fact I don't recommend it. As you heat the metal it will start to bulge, when this happens either spray water on it, or throw a wet rag on it. It will shrink instantly to just lower than it was before you heated it, and will be much stiffer. Do not keep heating the same spot or it will become brittle and crack.

I'm not saying godruler's method is wrong, i just use a different technique.