You can do the whole routine, but try this:
If the tank is not leaking, take it off the bike, empty it out and dry it. Wash with hot soapy water to get the oil out and dry it again. Then go down to the hardware store and get a box of 1/4-20 nuts. Pour them in and shake like you are making a martini. Empty them out and wash the tank out again. Re-mount and enjoy. The oil in the pre-mix will keep the tank from rusting any further. The rust came from condensation when the bike sat for some time.