sounds like the gas turned to varnish.... you can soak the tank with brush cleaner to try and disolve the varnish, wash it out with clean water and then get some Por15 Metal Ready, fill the tank with that and let it soak, periodically swishing it around, might take a day or two depending on how badly it's rusted, after the tank looks clean rinse it out with water and blow it out good with an airhose followed with a coating of Por15 Tank Sealer..
I work on alot of project motorcycles with that same problem. The cheapest way out is to get a bottle of The Works lime & rust remover with the blue label, fill the tank 1/2 way up with water add the rust remover & fill the tank all the way up to the filler tube with more water. Let it sit a day, shake it often, pour it out and blast the inside out with a pressure washer. 20 yr old Honda tank sparkled when done. It helps if you have a hose syphoning off the water in the tank while you are blasting it.
