"Works" toilet bowl cleaner. Does wonders. Just get a rubber plug/cork from Home Depot and a bottle of this stuff from Walmart, for like $2 or less, and cork and fill your tank. Top it off with water and let it sit for 1-2 hrs and check it with a flashlight. If there is still rust, let it sit. I put in some nuts inso I could shake it up and i put on the gas cap to flip it over and get the top side too. After you empty it, rinse it thoroughly with water and then coat the inside with wd40 or whatever lube you are comfortable with using, unless you can put it on your bike and fill it with gas.