When working with gas tanks, I pipe exhaust from a running vehicle into the tank until the tank is hot to the touch. An old, old timer told me that the carbon monoxide eliminates the oxygen and the explosion hazard. (he use to weld them with exhaust running into them, too - lived to be over 90)
Then I used a few lengths of chain and tumble the the tank to loosen the rusty stuff. Chain was easy to remove. Rinsed it out, dried it in the sun and used gas tank sealer. The sealer set tacky at first - so I coated the tank sending unit float with vaseline to keep it from sticking to the bottom of the tank before I got the gas in. Been working great for 12 years.
