This is a most useful thread, I have been 'peeing' about with my TIg welder for the betterpart of a day trying to get some settings to help repair my Series 3 bodywork, so far Ive learned from my own mistakes that cleanliness is key and its real easy to get a big hole very quick !... oh and patching that hole is really tricky. The initial job was to repair two small holes in the bonnet - drilled some time ago when it was dented and I think the 'repairer' was trying to pull the dent out - having removed the frame and repaired that, it was simple enough to beat the dent out and back into shape - just the holes left to sort. I do hope the OP adds more to this thread.