It also works well as a soak to remove the rust from inside gas tanks. Seal up the tank, pour it in through the filler cap and seal the tank. Slosh the acid around, occasionally, over about 30 minutes. Drain the acid, rinse the tank with alcohol and coat it with WD-40 to prevent flash rusting. Note; this will likely discolor the paint so I'd recommend doing this on a tank BEFORE painting it.
You can find it at Home Depot in the paint section (with the paint thinners, mineral spirits, etc). It's sold as Phosphoric Prep & Etch and runs about $15 a gallon.
