As a welder and fabricator by trade I would stand by the use of MIG for what Stuart has done. It is ideal for the enthusiast as Stuart has proven.
Alex, your barking up the wrong tree.
MIG enables smaller welds and less heat in the joints, multiple tack welds is essentially what you do or if you drill one piece and lay it on top the other piece and do a weld through the hole and seal it you are mimicing a spot weld in effect, although a spot weld is done by a pair of copper tongs clamping the two materials and passing a current through them and the work pieces.
TIG welding is far hotter and not worth the enthusiast playing with on jobs like this. It puts a ton of distortion into the panels.
Heliarc is a term from the stone ages and those in the trade call it TIG nowadays.
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