Just fill the tank up with water. Close the cap and vent tube. Turn the
tank over so that the area that you are going to weld is located at the
small air space left in the tank.
I have done this with Oxy-acetylene and occasionally I would get a
little burp at the crack or opening I was repairing. There wasn't enough
space for any gas residue to have any power.
Randy

"wallster" wrote in message
...
I have an old gas tank off of a motorcycle that needs a mounting tab
tacked
on to it. When I emptied the tank, the gas was real old and smelled like
varnish so i flushed the tank with acetone (rusty in there), then rinsed
it
out with soapy water several times. I dont think it would explode because
of
the age of the fuel that was in it and it was thoroughly rinsed but i'm
not
a big fan of blowing my fingers off my body so i'm a bit relunctant to mig
this thing. A friend suggested to go outside with the cap off and stick a
propane torch in the opening and see if it flashes. If it doesn't, it most
likely wont explode. Any comments before I muster up enough nerve to tack
this tank.

thanks in advance,
walt