White distilled vinegar (I bought it in two gallon cases at the local Sam’s Club for $4/case, and I needed 7.5 gallons).
Dawn soap
Something to plug the bottom of the tank (I bought a new sediment bowl setup from TM, as the one I had was starting to corrode badly, as I result I just left the old sediment bowl in place for the rest of this)
Some way to drain the tank back into the vinegar bottles once the job is done (I put a fitting on the old sediment bowl and used a rubber fuel line for this).
50 nuts or larger washers.
Baking Soda
8 Gallons of non-ethanol gasoline
Marvel’s Mystery Oil (MMO)
Process
Drain and discard any fuel in the tank
Remove the tank from the tractor
Wash the tank with some dawn and water to break apart any petroleum products left in the tank (they create a barrier for the acid to get to the rust, and make the process take longer). Get the soap out, but don’t worry about a few ounces of water left in the tank at this point.
Ensure the bottom is plugged, however you intend to do that.
Add vinegar until full to the very top. As the vinegar evaporates and/or has a chemical reaction it releases gas into the air that will cause any exposed metal at the top, including the gas cap, to rust very badly in no time. Use the baking soda to neutralize any vinegar that was spilled on the paint.
Each day, top off the vinegar in the tank (I found that I am bad at hitting the tank, so I kept the baking soda handy for this).
After 7 or 8 days, drain the vinegar back out into the vinegar bottles.
Add the cleanest bottle of vinegar back into the tank and throw in the nuts and washers, shake it around for 30-45 seconds (make sure you shake them all around the tank, these will knock any slightly loose rust off).
Dump the tank upside down and empty out all the washers and nuts (and loose rust).
Add some more water for another rinse, swish it around to pick up any leftover rust pieces, washers, or nuts, and then dump it again.
Add and then drain acetone to eliminate any leftover water.
Install the new sediment bowl on the tractor (Only if you are replacing it)
