I took Ron Covell's basic steel metalworking workshop at Roy Brizzio's stop in San Francisco. He covered all sorts of techniques for forming steel sheet, working out dents, etc. I would highly recommend this seminar for anyone interested in old cars.

Ron used gas and TIG for the welding in the class. One of the sample he did was to cut a 4x4" patch out of the center of a piece of sheet then reinstall it like a rust patch. He welded 1/2 of it with oxy-acetelene and the other half with TIG. His heat control was awesome with both methods, and there was much more heat signature on the gas bead but little warpage with either. He then proceded to metal work the whole thing smooth. The lesson? You can use either method for sheet metal. I still have the piece he did and can post pics of it if you want to see them.

Ron said TIG is his method of choice and doesn't use MIG much at all. Ron is a metal-working master of course, so he made it look easy, and he had top-notch TIG equipment to work with at Brizzio's studio. Not for a beginner but definitely can be done....