The page below is one way to remove rust from Fuel tanks and parts.
http://www.altelco.net/~jacil/clay/motorcycle/KElecSetup.htmlIt says to get Arm and Hammer Washing Soda. which is 100% Sodium Carbonate.
I also found 100% sodium Carbonate in the Pool section of the local Wal-Mart.
It is used to increase the PH of pool water. The Local Wal-Marts do not cary Arm and Hammer Washing Soda.
Just to help cover the rest of the tank concerns. Like so many other bikes out there I have a bike with Kreem in it and am now having rust getting into the fuel system. So now I must remove the Kreem before I remove the rust from the tank. The link below is how that can be done.
http://www.sense.net/~blaine/removingkreem.htmlI hope this post helps those that are having rust gas problems.