Even the products intended to seal rusted motorcycle tanks require some metal prep if the rust is anything but surface flash rust. Heavy rust is removed with a bottle or two of heavy duty radiator cleaner and a box of BB's. Obviously the tank has to be closed at the petcock and filler, or with a jerry can, put the filler cap and gasket back on. Then shake vigorously until the rust is dislodged. It does not have to be shiny clean, but you do want to eliminate heavy flakes adhering to the tank. Rinse and dry thoroughly. Then seal with the sealant product following manufacturer's directions. This usually includes leaving the filler cap off, distributing the sealant thoroughly around the inside of the can or tank, then pouring out the excess and letting it sit upside down to drain until dry.
It is not impossible that, if you spend the time with the cleaner/BB's that you can get the tank shiny clean. If you can do that, you can paint the inside of the can, or simply put it back in service, since most were not painted inside to start with.