Thanks to Noiseguy for this entry. (READ ENTIRE PAGE BEFORE STARTING)

"This worked so well I thought I should post it. I had a really rusty tank, not rusted through just filled with rust and old gas. It cost about $6 to do, for gas and C.L.R.

First, dump all the old gas out. Remove the fuel cap and fuel level sensor and shake it all out. Remove the vacuum fuel shutoff.

Reinstall the sender; plug the fuel outlet and fill the tank with clean gas about 1/2 to 3/4 full. Throw in some clean hex nuts (I used 25 x 1/2 in) and swish it around for 5 - 10 minutes. All sides, even top (if you can stand the spilled fuel.) empty the tank, and repeat 1 or 2 more times. You should be getting rust flakes out doing this; if you are not you are not shaking enough or the tank is clean.

Remove the sender and cap, shake all the gas and hex nuts out. Use a magnet probe to get any that will not shake out.

Put the tank in a container, upright, and fill with C.L.R., calcium lime rust remover. I dumped in 32 oz of this, and topped it off with boiling water. Be careful, wear goggles and gloves, and keep it off painted surfaces. Any acid-based rust remover will work for this. Let it set for 1-2 hours. There is a little bar across the fuel neck; you can use that to judge rust removal.

Dump out the acid and flood the tank with water. It will take a lot of water to get all the C.L.R. out. Put the hex nuts back in, recap the tank, fill 1/2 to 3/4 with water and resume the shaking routine. Repeat 2-3 times, until rusty water quits coming out and the tank looks clean inside.

Dry the tank out well. I filled it with some alcohol based gas dryer, dumped it and dried it with a hair dryer. Once the water is out, immediately reassemble the tank and fill it with gas. The raw metal will rust pretty quickly otherwise."

A follow up post by Jerry

"Your basic theory is great with only one exception. You say to drain fuel and then put in metal nuts to knock off the rust. That could be a fatal mistake. Sparks and fuel don’t mix.
Use water and glass marbles instead. The acid is good practice too as it removes the imbedded scale. When you drain the acid based cleaner, rinse several times and then dump in a handful of baking soda to neutralize and rinse several times again. Then air dry and use some dry gas. A hairdryer could cause explosion too even though you have rinsed with water. When tank is refilled with fresh fuel let it sit for a half hour open the drain on the bottom of the carburetor and release any left over water.
Scoot safely."