Stud guns are absolutely fantastic. You can almost pull a low spot to the point where no filler is required. They don't work so well on compound corners because of the strength of the metal with the dome shape, but adding a few relief cuts and welding them afterwards helps. Here is a shot of a few studs in my model a coupe decklid that I am working on now. You attach the slide hammer to the stud and start banging.
I have a friend who has one and I borrow his. It's a stinger stud gun and the kit is around 350 bucks if I recall. If you're planning on continuing to restore cars, it's probably worth the investment. The other option is to cut the spot out and weld in a new patch.
http://gallery.mac.com/patrick.plummer#100754/IMG_3875&bgcolor=black
