I watched an old guy weld a used fuel tank, he used a shop vac hose in the hole to evacuate the fumes and only welded a small amount at a time. He also did it in a big shop near a large open bay door so the fumes could build inside.
lol, used to that stuff; but I used a hose feeding exhaust from another running car to fill tank with no oxygen. A little flame outside the tank, quit flaming soon after. Today I shudder at what I used to do!