I clean a fuel tank by putting a few gallons of a strong mixture of laundry detergent and water into the tank and several pounds of pea gravel, then strap the tank to a rear wheel on my tractor and bush-hog a pasture for a couple of hours. Strapping the gravel-filled tank down in a pickup bed and running a few miles on a twisty 2 lane road also does a pretty good job. Getting the gravel back out is somewhat of a chore, but not impossible.
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!