nice collection of spares 
Wrote by Mr.Mohsin Ikram on VCCCP thread:
"There are various chemicals in the market which remove rust. I have used all of them and find a desi solution of sulphuric acid (namak wala tezab) the most effective. The only thing is that the tank would catch rust again if it is left empty. Perhaps you spray thick oil inside the tank with a spray gun (I have one with a long nozle)and clean it later when its time to fit the tank.
Berger Paints had once introduced a 'rust remover' but i dont see that anymore. But every good paintshop here has various brands of rust removers which should do the work for asad. I bought a 500 ml bottle for Rs.200/- some months ago."
another good recipe for Fuel Tank Cleaning by Mr.Mohsin Ikram:
"Get some caustic soda and mix with water...fill the tank up and let this solution sit for an hour..then drain this solution and rinse with water. Then take 2 portions of water and one portion acid and fill the tank up and leave it for an hour (depends on the intensity of rust). Drain the acid/water and if required repeat the process. Once satisfied with teh result, make a solution of water (one bucket) and baking soda (1/2 a cup) and fill the tank up and leave t for 30 minutes. Drain and rinse the tank with water. After a while fill the tank with some fuel and rinse it....drain the fuel and your tankis ready for new fuel."
