Mazak's recent post about making a forge from a gas tank - https://www.homemadetools.net/forum/...482#post246482 - reminded me of a trick that I have used to make used motorcycle fuel tanks safe for welding.

When I first needed to weld a fuel tank, which had had petrol in it, I tried flushing with water and later with water and detergent. However, there was always a small pop when I applied a welding torch, indicating that there was still a small amount of vapor in the tank. Moving on in time I learnt that if I ducted the exhaust of a car (diesel or petrol) into the tank for a few minutes before welding then there would be no sign of fumes remaining.

Pumping hot gas into a tank filled with fuel fumes may not seem like a recipe for a long life. So only do this after a water flush.
WARNING. Welding used fuel tanks is dangerous so do not do it, or you will get a visit from the health & safety police or coroner.