What is the reason for wanting to get the tank coated? It's a lot better idea to clean it properly than to try to encapsulate debris under some sort of coating that might (or might not) stay put. I clean gas tanks by putting a couple of shovels full of pea gravel into the tank and filling it with a strong Tide and water solution, then strapping the tank down in the bed of a pickup truck for about a week. A faster way is to ratchet-strap the tank tight to one of the rear wheels of my tractor and go out and bush-hog several acres of pasture.
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!