MOPED ARMY

That sounds like a good way to get most of the rust out. Buit even if you get all the rust out, understand that rust is an electro-chemical reaction... rust is iron oxide.

If you have iron, oxygen and water rust will form.

Iron is the tank, oxygen and moisture come from the air. That tank will rust again.

You must coat the (iron) tank with something to prevent air and moisture from contacting the metal if you want to stop it from rusting again.

Keeping the tank full to the top with fuel will do it. Kreeming the tank will also do it.

I used Behrs concrete and rust etcher. You can buy a gallon of it at home depot for about 12 bucks, fill your tank up and let it sit for a good hour or two. Drain the substance back into the container it came in and then flush out your tank with a lot of gas. This did the trick on my puch, theres still some debris down in the bottom of the tank but for the most part its really fine silt and doesnt come through till I put my tank on reserve. Every so often I have to clean out the carb but, like my other puch, I eventually had all that debris just get flushed out as I ran a lot of gas through and changed fuel filters often. The Behrs stuff is phosphoric acid, it acts like a sand paper at first, loosening all the rust then it disolves it. Its semi harmful stuff, gloves and googles are somewhat recommended, I would also try not to get it on the paint job.

Putting sand in a tank doesnt seem like that good of an idea, however lead pellets, like buckshot may work. Lead is soft, it wont damage or dent the tank. Water just seems like it would worrsen the rust problem.