How to clean rust from a gas tank
I have read several suggestions on how to clean the inside of a rusty fuel tank. Here is my method which came to me while working part time in a dive shop. As an avid scuba diver our air tanks must be cleaned and certified routinely.
Find a local dive shop that tumbles (cleans) air tanks. Purchase a quart jar of scuba tank tumbling media, it is a very hard substance in pieces that resemble mouse droppings. It is dustless and will not break down under harsh chemical conditions. No dive shops, its available on-line.
Drain the tank of any remaining liquids, fill about a 3rd full with water, my tank is 3.2 gallons so I put in a gallon, then add 2 cups of media, then shake, shake, shake! (I sometimes add a cap full of simple green as well to help with fuel resin build-up) You should drain through a screen so you can re-use this media, the media is large enough not to get lodged in tight corners, is dustless, washes clean with water.
If you don't plan on using the tank right away once the tank is empty and blown out with an air hose I take a cup of clean engine oil and coat the inside to prevent any flash rust, you can always rinse that with clean fuel prior to installation, or try some of the many coat and seal products which have been mentioned here.
Let me know if you have any questions, if its good enough for breathable oxygen, its good enough for our fuel tanks.