Auto body repair & detailing: replacing rear quarter on 55 chevy, quick zap, spot welder

Question
I have replaced rear quarter as per classic chevys instructions. The problem i am having is that when I get to the front by the doorjam the panel will bow in. I have cut that whole area clear again and everything will line up like it is supposed to be. When I start to weld it again the same thing happens all over again. I am using 20 guage wire on a mig welder, I have tried using air to cool the panel down as well as a wet rag but have no luck. Iam welding this panel about 1/2 inch at a time. Any help would be appreciated

Answer
What are you putting in- a 1/2 quarter skin? (Basically a great big rust repair panel) Are you bouncing around, or trying to weld with 1/2 inch stitches end to end? You need to bounce around. Also, get the panel basically tacked in with a couple dozen pencil eraser sized welds. You might try bracing the back side with some strap metal to help retain the shape of the face. You can also try to use Play-Doh as a heat sink around your stitches. Keep the play-doh moist. Go slow. Warpage is almost impossible to avoid when welding sheet metal, so expect at least a few waves, especially over an area this big. Your only other alternative is to rent a 2 gun spot welder, and spot weld the panel on- this type of welder doesn't have tongs, it just has 2 hand held guns- one with  a trigger, one without. You simply push both electrodes against the new panel to keep it flush with the old panel, and do a quick zap. Presto, 2 spot welds. Be careful, this can heat up and warp, also. Check progress with a straight edge. Then do a few strategically placed welds with the mig welder when you are done.