I have a TIG welder, but I'm not good enough with TIG yet to weld on the car. I've done some practice coupons like the one in the film using TIG, and patches can be done with lots less grinding. They warp somewhat more with TIG. The warp isn't hard to correct with hammer on dolly work. The key with either process is to weld in a short length of weld, then stop and grind and stretch the shrink back out. It's easier fix if you don't heat warp too much panel at once. Here's an example of a coupon with TIG with one end metal finished and heat warp corrected. I wish I could walk up to the TIG machine and do that every time. So far, I'm not consistent enough, so I'm using MIG.



The repairs I've done on the car are with a Miller 175 MIG, some using the Harris 0.03 "twenty gauge" wire and others with 70S6 0.03 wire. The Harris wire has an powder iron core that makes welding a little easier without burning through on thin metal and make it somewhat softer to work. Filling an old bodyshop pull hole is easier with the Harris wire. If you cut across welds made with twenty gauge compared with 70S6 using snips, you can feel the harder weld with the standard wire but not with the harris wire. That twenty gauge makes a softer weld and I think it would also be less likely to crack. Mostly the key is to grind the weld flat so you're not hammering on the weld much, but just correcting the heat warp. I haven't had any trouble with cracking.

Compared with TIG, the MIG creates a taller bead and that bead has to be ground nearly flat before you do any hammering. Then the warp needs to be corrected with hammer and dolly work just like in the TIG video example. The MIG welds don't crack if they are ground flat first. But as you say, you can't hammer weld it like you can a TIG weld (or a gas weld).

TIG would produce faster and better results because less grinding is needed. I would like to get good enough with TIG because it would be less work.