If you're restoring a motorcycle with a steel fuel tank it's possible that rust has formed on the inside of the fuel tank. Often, when you have an old fuel tank the liner is also breaking loose and making a real mess. Rust on the inside of the fuel tank is an annoying problem that many motorcycle owners have to deal with.
The best way to fix it is to put steel goose shot or 10 to 32 steel nuts in 'dry' fuel tank, wrap it in a blanket, put the wrapped fuel tank in a 22-liter plastic paint pail, and take it to the local hardware store. Then insert it in the paint shaker and shook it until it knocked all the rust and loose stuff free. After all the rust and nasty crap is flushed out, you can then re-coat the fuel tank with a good petrol gas tank sealer. Now I have an edge when it comes to the fuel tank sealer, since I made a fuel tank sealer years ago and still have some left. But that's not going to help you reading this article. There are several things to look for in a fuel tank sealer. First, it has to be alcohol-resistant due to the new fuel blends. Second it should be non-hardening after drying and remain flexible. Third, I personally don't care for sealers that use an acid wash and water rinse. This process can cause a fine layer of rust to form while the water is evaporating, and the sealer will stick to the rust and not the parent tank material. If this rust film detaches, it will allow the sealer to peel off after awhile, which will cause a mess as bad or even worse than the original problem.
If you decide to buy a new fuel tank, make sure it's coated or made in a way so that it doesn't do the same thing as the tank you have now. For example an original Honda Phantom TA200 fuel tank is coated with fuel tank sealer that is not alcohol-resistant. In that case buying a new fuel tank and using gasohol in Thailand will only help you for about 6 months to one year before the fuel tank starts to rust again. A new product available in Thailand is Metal Rescue, it can really help with rust in your fuel tank Rust Removal Made Easy