Gas welding is also a series of tacks, lest ye heat the panel to the point of warpage. Weld an inch or less, then hammer, then skip a space, and weld another inch or less, then hammer. The mig works a treat, when set up and used correctly. Way less hammering, and way less warpage. Also the back side ends up looking like the front side, with very little grinding needed. But, like gas welding, it takes practice. The main thing is both pieces must be CLEAN, and free of all rust and debris on both sides, before you even start.Well I'm about to upset a lot of people here. Welding sheet steel with a mig is butchery. They are NOT welds they are TACKS. A whole row of them that have to be ground down and Tacked over the missed bits. This is not welding. I know there's lots of "experts" on youtube doing this method, because it's quick and nasty. Covered up with bog. Have you ever seen the back side of these experts' welds ? NOPE. Because it's crap. Tig and Oxy-Acetylene takes practice. Both of these produce a far better result. I'm old school and every time I see this TACK method I'm upset and amused at the same time. They won't last long. Body work used to be an honourable skill. The craftsmanship was respected and appreciated by those that didn't have those skills. Now It's those that don't have those skills call themselves craftsmen. ( they are TACKERS and Cake decorators )
Bodywork is still an honorable skill, usually only found in exclusive restoration/hotrod shops, the collision repair business has turned into parts changers. Swapping out parts that I would repair, and pocket the part cost. I got out of the collision repair business many years ago, got tired of fighting with insurance companies, and adjusters, outlaw parts, and what all. I'm too old for that crap. I only do a select few jobs for a select few customers. Usually something interesting like 33 pickup, Model a ford, 85 Pontiac sleeper, etc.