I thought you guys were paying attention!!!!
The welding process caused metal to shrink. PERIOD!!!!
cooling with air, water, or waiting all friggin day isn;t going to have much effect on the outcome. I prefer to cool the weld seam, if need be, with compressed air. It doesn't give me any problems, and you are better off cooling quicker rather than letting the heat migrate further into the panel than need be. If you will always make butt welds and work the weld seam, only along the heat affected zone, your problems will be eliminated. There's absolutely no reason to use adhesives. That's a cheap, production body shop, get in/get out method. Do it right the first time and forget it!!