Simple rust removal without the need to Kreem or coat inside of tank with oil/WD40 (and your tank will NEVER rust again!)
The last tank I used this process on sat on a shelf in my garage with Ohio weather for two years with no gas in it and not a speck of rust appeared.
Items needed:
Tank (duh)
Large enough rubbermaid container to fit your tank, with the lid -OR- ability to seal the tank (cap, block off plate for petcock) MAKE SURE THERE ARE NO HOLES! lol
1-2 gallons of Milkstone remover (available from Tractor Supply Co) It's orange and is around $14 for a gallon jug. Milkstone remover's main ingredient is phosphoric acid.
Hair dryer
safety glasses and heavy nitrile gloves
I recommend removing large amounts of flaky rust first with screws or nuts with tank filled with water.
don safety glasses and gloves. This stuff will burn you.
Mix the Milkstone remover 1:3 with water. That is, one gallon of Milkstone remover to three gallons of water. That will make 4 gallons, which will easily be enough to fill a tank. Otherwise, mix up enough to completely fill/cover the tank when immersed in a rubbermaid container. Either way, make sure the tank is as full as possible. You may also invert the tank for one day to get any areas that you can't reach by filling or submersion. If you use a rubbermaid container make sure to put the lid on or it will stink up the whole garage; also it keeps kids and animals out of it.
I leave my solution in 2-3 days. When finished, drain the solution and rinse well with water. Then use the hair dryer propped in the filler hole to completely dry the inside. Two things have occurred- your rust is gone, and the tank is coated in a beautiful grey coating. This coating is actually the outside layer of steel being chemically converted to black ferric phosphate and it will prevent future rust, so leave it on! There is no need for any additional coatings like Kreem that can peel away over time. As for the outside of the tank (if you've submerged it) the coating can be scrubbed away to reveal the metal before prime/paint.
The only bad part about this process is that I haven't found a way to dispose of the used solution. What I did is pour it into a home depot bucket and left it, it also works good on removing rust on other parts and/or chrome. It will last a long time and will do quite a few tanks/parts.