Things You'll Need
You can add less water than required to the oxalic acid powder to make a paste that you can smear on the stone. But this should be left on no more than 10 minutes before scrubbing, since it will be more concentrated.
Rust on stone and flagstone can come from one of two sources: hard water that contains iron and is staining the stone or rusted metal that is sitting on top of the stone. Fortunately, while it may look terrible, your stone is probably not ruined. With some easy-to-make topical solutions and pastes, a little time and some elbow grease, you can remove the existing stains and prevent further rust formation.
Clean off the stone or flagstone. Sweep it clear of debris with a wire brush and rinse it off with a hose. Once the area is clear and clean, you can more easily assess the seriousness of your rust problem. Let the stone dry completely.
Treat rust stains with lemon juice. Do this on a sunny day. Saturate the stains with lemon juice. Make sure that they are completely covered. Allow the juice to sit on the stains in direct sun for five minutes before sprinkling them with baking soda and scrubbing them with the wire brush. The lemon juice will often lift the stains off the stone.
Mix up an oxalic acid (wood bleach) solution to tackle stubborn stains. Wear your gloves from this point forward. In the plastic tub, mix up your oxalic acid. This will involve adding a measured amount of water to the powdered wood bleach. You will need to refer to the packaging for exact measurements, since different manufacturers make this product in different concentrations.
Submerge the stone (if you are able to lift it) in the oxalic acid bath for 30 minutes. If it is flagstone, however, saturate a cleaning rag and lay it over the stains for 15 minutes. Remember this is a type of bleach, so it may change the color of your flagstone. Test it on a small, unnoticeable spot before you spread it all over your flagstone.
Scrub the stone again with the scrub brush and the baking soda. The oxalic acid will loosen and remove any remaining rust.
Rinse the stone thoroughly with a garden hose. Use lots of water so that the acid is completely diluted and removed. The baking soda will have helped neutralize it as well.
Carole Ellis began writing in 2004 for the "UGA Research Magazine." Her work has appeared in Growing Edge, Medscape and Doctors' Guide publications. In addition to medical coverage, Carole publishes a real estate newsletter called REJournalOnline and is the news editor for the Bryan Ellis Real Estate Letter. She has a bachelor's degree in English and graduate work in creative writing and plant biology.