I told Cam I was going to give away his secrets...He said "spread the knowledge". So here we go...
Welded with a Lincoln Precision TIG 275 at 50 amps, pulsing the pedal...
His reply in his own words: "As for the Cleco holes, there is a bit of a process...once the proper panel and joint fit up is achieved, the parts are tacked together at 3/4" intervals, generally starting from one end and working to the other. As the joint pulls together, as the tacks shrink, I hammer them back to bring the fit back to proper and continue tacking. Repeat as necessary until tacked across the joint. When finished, the joint usually takes just a quick pass to set the shape, if there are any discrepancies, which leaves it ready for welding.
At this point, with proper shape achieved and tension relieved, how the panel reacts to the weld is fairly predictable and minimal. As the heat is input into the panel only along the weld joint, the heat input and therefore the shrinking and distortion happen at a consistent rate, along the minimum area, without adding any peripheral distortion. Once the weld joint is finished, the Cleco holes are small and weld shut quickly with a blast from either MIG or TIG, and usually distort fairly lightly, needing only a couple of minutes each to correct. I have just found it easier and faster to do it this way as it allows work to be done one step at a time, eliminating variables that may interfere with each other, and give false or confusing info when reading the panel after it moved around from all the welding. They can also act as little windows to locate the dolly, if you're in a spot where locating your dolly be feel is being less than agreeable.
Simply put, it keeps distortion to a minimum during the weld of the joint, which makes finishing faster and easier on both steps. The usually distort very little when welded shut while in shape. Many times they will move like a B..tch if the rigidity is put back in the panel first."