The tanks came originally with bare steel inside, and this lasts pretty well. If it doesn't leak (ie no spots have rusted right through) you can just clean it and use it.
Use a phosphoric acid solution, hardware stores sell it as concrete etch to use before painting a garage floor. This will attack aluminum, so remove the petcock and use a rubber stopper. Pour some in, swish it around, leave it an hour or so. Adding some pea gravel and shaking it will help lots. Don't get the acid on you or your clothes though! The rust is always worse at the bottom where any water in the tank collects.
Rinse well with plain water and dry thoroughly. A hair dryer in the filler woks, or just leave it in the sun for a while.
If your local gas contains ethanol then you will never have free water pooling again, if not add some gas line antifreeze occasionally and expecially for long term storage if you leave gas in the tank.
Use an inline gas filter. There will be little specks of crap in your gas (rusty tank or not) and they tend to stick in the carb float valves and cause overflowing.
If you actually have leaks, use a Kreem or POR-15 kit. Both work the same and there are proponents of either one over the other. They coat the inside with a hard plasticy goop, sometimes this peels off or dissolves and causes way more problems that a plain steel surface would.
