My first foray into welding gas tanks was as a teenager. I gas welded so many patches on the tanks on my ranger with coat hangers that the tanks were more patches than tank. Every time you'd get a new patch on you ended up with more holes. Being gas and all, I didn't dare weld without water in the tank, and I think that the patches being dissimilar thickness to the parent metal helped cause the tank to crack more as it cooled.
I had a hole in one of the tanks on my latest van, and simply loosened the straps enough to keep the residual diesel off to the far side, heated with oxy/acyt, tacked it quick with some steel rod and that was that.
Diesel is so much nicer to deal with than gas, fire hazard is still there to some degree. Coat hangers are cheap.... But if you have anything larger than a small hole, i would pull the tank so that I don't have to weld standing on my head. Rust spatters everywhere.
At one point I got frustrated with the thin walls of fuel tanks and made my own out of 16ga steel,..but I wouldn't recommend that unless you're a glutton for punishment.
As I get older, I get more paranoid, and the thought of a tank that might spring a leak on me at the next pothole is something I am less inclined to tolerate. But if it's a vehicle that's on its way to the boneyard in the next year or so, I'd do it.
I have never tried brazing, I assume the rational would be either an attempt to keep the parent metal from distorting or crystallizing?
....
