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Steve: You have about three things to remove in that tank: dirt, gelled/varnished gasoline, and red rust. Couple of things you don't want to do: 1)Disolve any plating (like zinc) from the inside of the tank 2)Make a near rust through hole a real hole. 3) Use acid so strong that you generate a significant volume of hydrogen gas... That's why I am not a fan of muriatic acid (a comon blend of Hydrchloric and Sulfuric acids) except to clean masonry. Lately I have been doing the following: First, if accessible, insert a small length of chain to use as an agitator. Be careful to remove any tank gauge floats if applicable. If I think it's just old organic sludge in the tank, like boiled off gasoline, try acetone first (cheaper), then try MEK (Methyl Ethyl Ketone). MEK will do the job, but be sure to use it in a well ventilated area, eye protection, ....yadda, yadda....you know the drill. 2 quarts should do the trick. If dirt remains, it's probably water soluble, put a cork in the bottom, and fill with hot water and lye. Soak over night, drain, then power wash every where the nozzle will reach.
If you then see any visible rust in the tank, go to the discount auto parts store, and buy some metal treating stuff. It's kind of greenish, and it is now sold by them to treat (convert) free rust into the black iron oxide and iron phosphate. Now this stuff is dilute phosphoric acid, but remember, not all acids are alike. This stuff will eat metal, but at it's low strength, about all you will get is a good etch. It WILL turn the old rust black, and that rust patch will then be chemically inert. The tank must be oil free so the acid can get to the rusted metal, so I like to wash the inside of the tank with hot soapy water, or purple cleaner to remove the oily residue from the MEK and slop. Then slosh the phosphoric acid around. Be sure to wear protective clothing and eye protection, etc. (It will bite you nose !). Let it sit for at least 45 min or an hour.
Now, rinse the tank with hot water, or even better, blast it with a power washer. Though they say it will dry to a sandable, primable finish, I like to remove it. If you do this, it will shine, and you won't have to worry that you removed too much base metal. After you wash it, slosh some MEK in it, and most of the water will pool out the drain. If you are not going to install it for sometime, slosh some diesel fuel it in to oil the fresh metal surfaces because the MEK will have stripped all oil from the metal pores. BTW: I treat my freshly sandblasted parts with this "Phosphatizing" acid, then power wash, dry, and prime. The acid kills any rust I missed, and disolves any remnants of silica from the sand exploding against the metal. Frank
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