You can wash out a gas tank as many times as you want but when the heat is applied and it gets red hot there is still gas in the fabric of the metal and fumes will build up and ignite . If you are lucky and all the caps are off you may end up shook up and singed eyebrows or burns on your face or hands.
If you go to welding web.com you can read many stories of accidents.
A refinery accident happened this same way in California when a welder used a hand grinder to cut a metal tube that had previously had some petroleum product in it and 4 workers were burned to death.
The forman told the company he did not think it was safe. The company management said if you don't do it
we will get someone else to do it, Otherwise they threatened to fire him.
A high school student was killed this same way when he took a 50 gallon drum and washed it out and then took a 4 inch grinder with cut off wheel to it and it blew up and killed the young man.
He was in a high school shop class and the instructor was not informed about the danger.
The steel drum had previously held coconut oil in it, which had some petroleum based product .
On a Roadstar Adventure.