Well, I had to weld a tank, it was a portable air compressor w/5hp Briggs engine. The tank was leaking where the wheel brackets and the front stand brackets were welded to the tank.
I knew nothing about the fuss about welding on tanks, I just cleaned the area around the cracks some, welded the cracks and brackets. No pressure test, only a leak test. Remounted the engine and compressor and reinstalled the whole thing back into the pickup bed and used it for many years. Only thing that changed was the pickups and replacement engines and a compressor.
I suppose that tanks over 150 to 200 lb rating could be another story.
Dixon