I started using this process and have had good results cleaning out some pretty disgusting tanks. I start with Lacquer Thinner to thin to dissolve the varnish. I fill up at least half full and soak the tank for few days, rotating to make sure I get everything.
I drain the Lacquer Thinner back into its container and fill with Metal Rescue to treat any rust in the tank. Metal Rescue works best over 68 degrees Fahrenheit. How long the Metal Rescue remains in the tank depends on temperature and amount of rust in the tank. I picked up both items at Home Depot.
After draining the Metal Rescue back into its container I use a pressure washer set on a low setting (higher than car wash mode, but not high enough to damage the tank) and the tanks have come out very clean.
After pressure washing I coat the inside of the tank liberally with WD40. I then drain out the WD40 residue and slosh with gas to make sure I get it all.