Hi Bernd, I've read about gas welding aluminum, and I've tried it a couple times experimentally, just a couple pieces of scrap together, and while someone who has experience with it may be able to do wonderful things and perhaps things tig won't, I am not that man. I will be tigging the tank, I've got a nice welder that was top of the line when I bought it fifteen years ago, I just had to replace the water cooled torch, and the new torch is perhaps a third the size of the one it replaced while having greater power capacity. The old torch was a freebie, and probably sixty or seventy years old. I like polished aluminum on bikes, and have made quite a few tanks, both fuel and oil, for many other people, but this tank has been on my mind for more than thirty years, the spinning blank has been on the face plate for about five years, glued up but unturned, I ran into a problem in what I was doing the other night, didn't have much time, saw the never cut blank, grabbed a sheet and a chisel, and in about twenty minutes turned the rough blank pretty close to final dimensions all free hand, I'd been kind of putting of turning it until I made a template for the radius, couldn't find a decent piece of carboard, so just decided to wing it, and I think it pretty closely approximates a hemisphere. Close enough to make a template after the bondo and before the finishing with files, paper and the like. Did you ever find that after you made something custom for yourself, you didn't want to make a copy of it for someone else, who sees it, and wants one "just like that"? I'll take pictures as I go, just to make a record. When I got hit with a virus a couple months ago I lost thirty years of pictures including all my overseas pictures, and all my pictures of my cross country trips, putting it on the internet means its always out there. my best, mad jack