I have cleaned several gas tanks using muriatic acid solution, available cheap at any hardware store. I have since found out that Phosphoric acid is better because it does not promote rust and actually leaves a protective surface by some chemical reaction. Apparently, it is axailable at farm stores because it is used to clean stainless steel milking equipment.
If you are using the bike regularly, you do not realy need the kreem coating, fuel seems to prtect the tank, especially if you are using gas/oil mix.
You may want to surf some of the vintage motor cycle sites, they talk about this a lot.