Don. I think what steve is referring to is metals tendency to have a boatload of internal stresses in it. From casting forging stamping etc the metal molecules are in no way relaxed. Thats how a hammer and dolly are so effective at shaping metal. One adds a little force while the other gives the metal something to whack into, dispersing that force in turn to relieve the internal stresses. So the result is gently forming the metal. At work i tig weld vacuum tight fixtures, and clamping something straight usually ends up crooked as heck. When you heat up metal or cut it, you do the same thing a hammer does and gives those internal stresses an easy path to relax. Thats why metal warps when you weld it or sand blast it, grind it etc. We're so often and easy to zip screw it all down, and start at one corner, by the time we get to the other end of the panel we need screw drivers pulling the flange in or out to get it inplace. If you massage a panel so that one simple zip screw in the top center of the metal will hold the whole panel in place with minimal pushing and pulling, and all the flanges lay naturally where the factory wouldve spot welded them, when you heat it up to weld you have alot less warpage and fighting/finishing to do. Thats why full quarters are better. When you patch with a skin you have two pieces of different metal with different internal stresses both relaxing in different directions so you have a ton of body work to make the weld flat and look nice under a shiny paint job. On a full quarter you have the same warpage but on flanges and lips that were designed to be hidden anyways at the factory. But massing the panel so those flanges are flat and snug without clamps, theres no artifcial internal stresses somewhere out in the middle of the panel because you clamped the end to pull it in. Causing a buldge or dip you have to fix later. Like holding two edges of a sheet of paper then taking one hand and moving it a tiny bit closer to the other hand or twisting one hand just a hair. Youll see that a tiny movement creates a ton of "extra paper" between your hands. Thats what the clamps do