Welding help & advice

I was a welding/quality assurance engineer for 30 some odd years. Wouldn't want to get in a dispute with a ME but in fact MIG welding does not weaken anything. The metal mechanical properties change a bit with the grain structure in the heat affected zone, but in the case of sheet metal you'll never see it. If you really believe MIG welding weakens the metal you better not drive over any bridges.

You can spot weld metal together simply by drilling a hole in one piece of the overlay and then welding from the base material to fill the void of the hole, it's done all the time. Care has to be taken so you don't apply too much heat when filling the hole, i.e., weld and wait, weld and wait.