I just read an article in Motorcycle Cruiser magazine. They poured WD40 into a tank with a handful of 3/8" nuts, after removing everything and blocking the holes. Shook the tank for about 15 minutes, then flushed it with gasoline(or brake cleaner). They say it will remove surface rust and that the tank can be put back into service after making sure the screen is clean. For more severe rust, they recommended coating the tank, but didn't endorse any brands.
I had a buddy take his to a friends radiator shop, and they did it for him. Not sure which coating they used though.