Originally Posted by 41'srfun
A gas tank could set completely dry for several years and still have enough fumes in it to blow your head off if you were to cut or weld on it. The safest way to approach it would be to have it steamed out or thoroughly cleaned at a radiator shop and checked with a hydro carbon tester or "sniffer" to make sure it isn't still hot before cutting it.
There are a few ways to get by without going to that much trouble, but I don't recommend them to anyone. It just isn't worth taking a chance on having it blow up.
I'll X2 this. I was going to modify an old Harley tank some years ago that had sat empty on a shelf in the shop for years. Just to be safe I filled it with water. The surface of which caught fire as soon as the torch was brought over the filler bung. Which shows you that something flammable whether fumes, or microscopic amounts of fluid stays in the recesses or in the metal itself for a long, long time.