I used to occasionally pick up cars of various breeds that had been sitting for years. The tanks would be rusty. I would remove the tanks and feed a length of chain into them along with solvent before sealing them up.

I would then tip and roll the tank over a period of days before dumping the contents into a container. Then a series of rinses before letting it dry completely- as in no chance of any moisture remaining inside.

At the time I used a gas tank sealer from Hirsch Auto. It was similar to contact cement. I put a quart in the dry tank and then sealed it up again. Then the tipping and rolling routine started again until I felt the entire inside surface was coated. Then it was just the task of draining the remaining sealer and allowing the material to set up.

https://www.hirschauto.com/ALCOHOL-RESISTANT-GAS-TANK-SEALER/productinfo/GTS/

In the years that have passed since I was doing this the products have probably improved and there are more suppliers as is evidenced by a search of "gas tank sealer"

Randy
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Any twenty minute job is just a broken bolt away from a three day ordeal