[bojoe]

I generally use a three pronged attack against the varnish, starting with cheap chemicals and working my way up until the job is done.

First you need to determine if it's just varnish and decomposed gasoline, and not varnish plus gravel, dirt, bird feathers, straw, nests, and rust, etc, like one finds on in an old tractor tank behind a barn with no cap.

To dissolve varnish, my favorite solvent is Methyl Ethyl Ketone, or MEK. You can find this in the paint/thinners section of Lowe's/Home Depot/Ace Hardware. You'll need several quarts depending on the size of the tank and the amount of varnish in there. Fill the tank part way, swish it around, and then let it capped for a while so it can work. Now you swish, drain, repeat, and maybe use a few nuts that can be retrieved easily.

If you have non-petroleum contaminants in there, hot lye water will really do the job to emulsify that stuff into a water-soluble paste that can [probably be removed with a good hot water rinsing. Using a power washer will also help speed the job. You can also repeat this process of hot lye rinsing, but be careful that you don't harm the paint on the outside of the tank if that matters. If it does not, fill it with a cork in the bung, and cap it and let it sit overnight. Most of the petroleum will turn to a type of primitive soap, and the dirt will deteriorate into simpler sediment, both of which can be rinsed out.

The last chemical I use is Phosphoric Acid. This will convert any red rust into an inert form of black oxide that will be harmless to leave in the tank. Now, rinse, slosh, and repeat, until you can no longer see any rust. You'll want to rinse with plenty of water to get the foamy, soapy residue of the acid out, but don't worry about the discoloration left behind. That is a phosphate coating that will help inhibit rust in the future.

To finish it, slosh it with some lacquer thinner and that will work to drive the water out of any creases or seams. If you're not going to use the tank right away, I like to swirl a few ounces of diesel fuel to oil the inside so no further rusting can take place. If you want to use it right now, rinse it with a few ounces of gasoline and let this run out. It will take any last vestiges of water with it and you should have a pretty clean tank.