I skipped ahead bit by bit in places, but this was an interesting video. Maybe especially about the shrinking disk, because that's something of a more recent development, something I've heard of, but hadn't ever seen used. When I was working on the dents in the roof of my 46 Plymouth (someone had been on top of it at some point in its past), at first I wasn't planning to remove the headliner, and so I could only work from one side. I had it painted before I later pulled the head liner out, but had managed to get them all out except for one, where I collapsed the rim of the dent by working in one area too much (had to use some body putty there...). I was using glancing blows, working back and forth and around the dent, to "pull" the metal back up.
I have also more recently used this method on flat aluminum panels that were stretched during the process of punching a lot of small holes to form a ventilation area. (Not on a car, but on something else I was getting punched out on a turrent punch at a local farm machinery manufacturing plant.) In that case, I laid the panel (16 gauge aluminum) flat on a piece of heavy plywood, and alternated front-side and back-side, back and forth, to shrink the metal enough that it could lay flat again. I also had to work very carefully on this, because since the aluminum was not to be painted, I didn't want to mar the finish of the panel. (The area that was stretched was just around 3 5/8" diameter, with a whole slew of 3/32" holes, in a grid. Then a 92mm 12 VDC ventilation fan was mounted in this area. I suspect that aluminum stretches more under those conditions than would steel, but I wanted it unpainted, as well as the additional material thickness w/o the weight of steel, and steel would of course rust.)
