the gravel well act like an abrasive and scour out the rust. I usually shake it around for five minutes and then over an empty garbage can turn it over and start shaking the gravel back out, this is the most annoying step in the process Once you can no longer hear gravel bouncing around in the tank turn it upright and whack all around the outside of the tank to make sure that no bits of gravel have gotten stuck. Rinse the tank out with water and let it dry completely. Repeat the above process until the tank is sufficiently clean and then install an inline filter to catch anything you missed. I have done this successfully on a few motorcycle tanks and it is pretty effective. I'm sure there is a chemical you can use that would work as well but I like this method as it is relatively clean and cheap.
