"JohnM" (clip) You can fill it with CO2 if you're feeling squeamish, being
on the safe side is a Good Thing.
^^^^^^^^^^^^^^^^
No matter how many times you rinse it, or how long you let it stand empty
with the cap off, it is NEVER safe to assume that all the gas is gone. It
creeps into the seams, and then comes out as vapor when the welding heat
reaches it.

I once had to weld up some cracks around the fuel valve on the bottom of a
Harley tank. I filled the tank with water, put the cap on, and turned the
tank over. I thought I was doing fine, but this is what took place: A
little stream of water was coming out the vent hole in the cap, so the water
level dropped very slowly. Gasoline inside the tank (there must have been a
little left) floated to the surface of the water. The welding heat set it
off, and it exploded, blowing the cap off. A solid column of water came
out, hit the bench, and got me and everything wet. I am not religious, but
I took this a message not to do it this way.

I have heard of a welding shop which runs the exhaust from a small engine
into the tank for an hour or so, and then welds. They're still in business,
so I guess this is safe.