I have cleaned gas tanks by putting a shovel full of pea gravel and a few gallons of laundry detergent and water inside, sealing up the holes, and strapping the tank to one rear wheel of my tractor. then I go out and bush-hog a couple of acres of pasture. If you don't have a tractor, strap the tank down in the bed of a pickup truck and take a few hours' road trip on some twisty back roads. Don't have a pickup? What kind of stovebolter are you, anyway?
LOL!

There's a product the aircraft industry uses to seal fuel tanks known as "slosh sealer". It's a liquid that is poured into a tank which coats the inside surfaces and glues any debris to the inside. That stuff might be more effective than similar sealers sold for automotive use.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!