See Electrolytic Tank De-rusting for an acid free alternative.

Patrick Geerling's posting of the best chemical way to de-rust a tank and figured I'd include it:

Here's a method I found to clean your tank. Took it from: http://www.captain.norton.clara.net/cnn2sec41.html

Seems pretty plausible:

There is no reason not to use the old-fashioned brit-iron way of derusting. This involves using an acidic solution to remove the rust. I enclose the info given to me some years back below. This will remove the rust and stabilize the surface, preventing it from rusting in the foreseeable future. The acid treatment is OK, but even better is Wrangl solution, used here in this lab (corrosion lab) to remove oxides from steel:

1000 ml HCl ,specific gravity: 1.19
20 g Sb2O3 Antimony Oxide
50 g SnCl4 Stannous Chloride

Throw a buch of old nuts and bolts into the tank, pour in the solution, and shake the tank vigorously for 1-25 min at 20-25 C.

The data is taken from ASTM, treatment designation C.3.1.

Rinse out with *lots* of water, alternatively one second treatment with 2 spoons baking soda in water. The chemicals are not toxic in diluted quantities and may be aquired at the chemist, but do not drink the stuff.

Standard precations when working with acids is needed, as it is when using KREEM and other treatments too.

Put in gasoline as soon as possible after cleaning and go for a ride
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Personally like the last line the best.

Now if you are going to just use an acid, phosphoric is the best choice. While slower than muriatic (HCl), it won't start rusting immediately after treatment despite using a neutralizing wash. The extra chemicals in the above formula address this. Many that have acid cleaned their tank have been dismayed and the rapid formation of rust before their eyes afterwards.

Jeff "The Mountin Man" - '86 MaximX

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(editor's note)
Mention is made of a product called KREEM, above. It should be noted that many members have reported failures of the KREEM lining, especially if the preparation is not absolutely perfect. Suggested alternatives recommended by members include:

POR15

Red-Kote

Eastwood Sealer