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hi, this is john from san jose, ca. I am trying to learn how to post pics. hope this works.

My dad has welded a bunch of gas tanks. He would attach a hose from a car's exhaust pipe to the tank filler. As long as the tank is getting fed that exhaust it won't explode. It always freaked me out, but he never had any problems.

Now I'm curious to see your pic. (Make sure it's not too large, that's a common cause for problems attaching.)

There's a body shop in my town that's been doing that for decades. They have a flex metal pipe, run it to the filler neck, close all the holes, and let the engine run for about a half hour. According to the owner "never had a problem, just make sure it's been dry of gas for a couple days". I've never tried it though.

You can get a tank of any noble gas (argon, helium, etc) and trickle into the tank while you seld. It will displace the oxygen, just like the exhaust gas above. No Oxygen=No Boom!

Hmmm, I have always just washed them out with a good soapy water solution then smelled inside the filler hole and they never smell like gas so I welded away.

.....And then there is the time tested approach of simply tossing DRY ICE in the tank and welding away. That said, I always used the Soap & Water approach with NO problems.

Best,
Terry

You could fill it up with water.

water does not work, but I can't remember why. believe it or not if it is full of gasoline it will not explode, but is very difficult to weld that way and you will have fires outside the tank. the exhaust gas is simple and only requires a flex hose and some duct tape, most people have on hand, water has too much oxygen do nto attempt, john

Dry Ice.

True. I did on a 65 panel van. Filled it with water until I had just a small pocket of air. Ran the torch past it to ignite off any residual gas and welded it up.

EDIT: Where are my manners