I had varnish type crud on the bottom of one of mine. I took regular carb cleaner and poured it in the tank and let it sit for a few days. Then before I drained it, I put in a bunch of 3/8 nuts and bolts. I shook it around a bunch and then drained it back into my carb cleaner pail through my strainer bucket. It took all the old crap out and cleaned my nuts and bolts too. I just pressure washed the inside of the tank, blew it as dry as I could and then took some wd40 and poured a quart in it and shook it to coat all the surfaces and remove any moisture. drain it and away I went.
