Years ago I had the same problem on a different bike. There is (or was) a kit out there that is a 3-step process. First you rinse out as much junk out of the tank as possible, then you add a chemical cleaner. After rinsing that around for a while, you drain it out and put some alcohol in and slosh around to remove the water/chemicals. Then in the last step, you plug the fuel petcock hole and filler neck and ad a white sealer into the tank. Then roll the tank around in all possible ways to coat the entire surface of the interior of the tank. Mine worked really well, but because it is a messy process, others have not had good luck with it.
The stuff is called KREEM as lotsokids said its a 3 step deal and is available at most dealer parts departments. It is a little messy but it does work good. What is the extent of the rust in your tank. Heres a little trick for minor rust in the tank. You will need abouot a half gallon of gas and a hand full of BB's. drop in the gas and BB's in the tank and shake it around ALOT the bb's will knock off the rust and shine up the inside real pretty. *NOTE* count the bb's before you drop them in and when you take them out to make sure you got em all.
i cleaned my tank using the BB method as well; it worked very well. i didn't use any cleanser, just BBs and distilled water. i rinsed the tank out, then added the BBs and distilled water and sloshed those around in there, drained completely and rinsed a total of 3 times. cleaned the tank right up!
counting the BBs is a crucial step in this process! you don't one or two of them in there swimming around while your riding and work their way through the fuel lines. that would totally blow.
Yeah, plus if you use steel ones they will eventually rust and clog up the whole carb system. If you have a hard time getting them out and you used steel ones, get a magnet on a string and get em out that way.