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Tyler B: 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: If I think it's just old organic sludge in the tank, like boiled off gasoline, then try acetone first, 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. 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 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 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, if the tank is reallly rusty, throw a short length of chain in there as you slosh the phosphoric acid around. Be sure to wear protective clothing and eye protection, etc. (It will bite you nose !). 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 clean 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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