[NYCSTRIPES]

If the tank is rusty inside, you really should neutralize the rust before trying to use that tank.
I have used a product called KREEM. Two part system. One removes the rust , the other coats the interior of the tank. Drawback is that you don't want to get that crap on your paint and sometimes the sealant freezes the fuel float inside the tank that shows your fuel level if that is the type of tank you have. If your bike does not rely on this system to show how full/empty your tank is, that's good.

If your tank is just loaded up with debris, not rust, you can flush it out with soapy water.
Ken