[Harpkatt]

When you apply heat to the panel, the following cooling shrinks the local area. uneven heating and its following cooling are what cause panel distortion. when you weld if you add too much heat the surrounding metal is sucked into the weld pool, this is undercut. on thick plates it's very easy to see this effect. on thin sheet, you basically burn right through. Tig welds are softer, more easily hammered on. You typically hammer a weld back to flat to get the metal to its original thickness and undo the warpage. This takes much practice. Mig welds are much more brittle. hammering the panel can crack along a mig bead. Tig will give a much better finished product, much much slower up front but less finish work. Mig is less work up front but grinding is harder and if you plan to hammer and dolly the panel to straight much more difficult. If you are okay using a little more bondo then mig is the way to go. Tig is much harder to master. I suggest doing a lot of practice on thin steel before ever touching the car with a tig..or mig for that matter...Ask me how I know.

Tig also has to be stupidly clean to weld. if you hit a pocket of rust or cut corners you will burn through the panel.