I was recently cleaning my tank. and what i found to work to loosen up all the loose stuff and some varnish. I poured in about a 1/2 gallon of paint thinner and i dropped in a steel chain and is shook the tank and it got a lot of stuff out. you can repeat that a few times. but I am planning on replacing my tank once my car is finished becuse there is still alot of sediment in there that i could not get out
