I used a body dolly and works good for some applications. Used copper for a lot of things. But mainly use aluminum. I always back up any sheet metal welding with aluminum, when possible. It will draw the heat out and keep metal from warping and drawing up. When welding a hole, just drop a spot on at a time. Dont run a continuous bead. Too may times you will burn thru and will all fall on the floor, or worse your foot. Or its built up on top and just grind thru it when cleaning up. I like to tap the edge of hole in slightly and then it will tend to fill in more flush and still have some thickness. In places where exterior panels are butted together, I will weld from the back, using aluminum on finish side. Then almost no grinding is needed. And no warpage. Works very well if able to do this way. When welding spot weld holes like floor pans, I have someone to push against the blind side(new panel being welded in) with a piece of aluminum round stock. I can get 100% penetration, no burn thru, no warping, and a perfect "appearing" spot weld finish. No grinding needed.