I decided against trying to join my rear wings back together, mainly because one of them incl. its B post needed work, and just bought a new pair. However, having been switching between TIG and MIG for the last 10 years and appreciating the pros and cons of both, I think I would use MIG for this job and join with a series of tacks. Here is my thought process: tacking with the MIG is going to be the easiest way to avoid distortion, covers more distance per weld and uses a lot less gas. Tacking with the TIG is slower because you have to feed the rod in by hand and wait for the puddle - ultimately, this generates more heat than MIG tacking, and uses a lot more (expensive Argon) gas. Of course, you can lay down nice short weld sections with TIG, even without the rod, but this is much more difficult to get right when its not flat on the bench I find, and creates a lot of heat that needs the air hose to cool it. MIG tacks are going to be your friend on these vertical panels! Use plenty of small neodymium magnets to keep the panels flush initially - the heat will force the panels to become less flush, but you can fix this between tacks with a sharp flat tool or panel hammer. If your sheet metal is completely solid along the edges, which I assume it is, this will allow you to increase the current and the MIG tacks welds will be pretty flat and will only need a tickle with the grinder afterwards. I find that the copper plate tends to prevent good penetration at the back - you really need that weld on the back, else your tacks won't join together and you will see the panel edges here and there.
That idea proposed on Youtube seems to work OK, but having watched it carefully, I still think personally that templating the repair piece in cardboard and making the repair piece fit the hole perfectly e.g. using the flat side of a cutting disc is still the way to go. Neodymium magnets are your friend here, again, to hold the repair panel flush in the hole. The Youtube idea will keep an even gap though while you join the panels, but you have to faff about cutting the tack welds to get the panels flush again. Magnets dont hold the gap even, but you can fix the gap with tools as you initially tack the panel in place. Just my opinion...
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