A friend of my son who was short on cash showed up at my son's house one day with a dented gas tank. He'd had a mishap and a bent handle bar had dented his gas tank inward about 2 inches. My son's friend asked him if he had any ideas about how to repair it, since he couldn't afford to have it replaced or professionally repaired.
My son said he thought so and proceeded to drain all the gasoline out of the tank. Then he poured about a half a can of ether starter fluid into the tank, picked up a match with a very long set of needle nose pliers, lit it, and held it over the tank mouth.
Bang! A fireball shot about 10 feet into the air out of the tank mouth - instant tank repair.
My son says the dent is gone. And, if you squint your eyes a little, the tank looks pretty much like it did before his friend's accident. But, it's a little fatter now and my son recommends a professional do the work if you want a more predictable result.