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 patio probably isn't 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. Tannin stains from leaves and twigs often look like rust and can be removed using the same techniques you use for rust, even though they aren't the same.
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How to Remove Rust From Stone or Flagstone
Step 1: Clean the Stones
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.
Step 2: Apply Lemon Juice
Treat rust stains with lemon juice on a sunny day. Saturate the stains with lemon juice. Make sure that they're 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.
Step 3: Prepare Oxalic Acid
Mix up an oxalic acid (wood bleach that also removes rust) solution to tackle stubborn stains. Wear your gloves from this point forward. In the plastic tub, mix up your oxalic acid by adding a measured amount of water to the powdered wood bleach. You'll need to refer to the packaging for exact measurements since different manufacturers use different concentrations. 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's more concentrated.
Step 4: Apply Oxalic Acid
Submerge the stone, if you can lift it, in the oxalic acid bath for 30 minutes. If it's 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.
Step 5: Scrub the Stones
Scrub the stone again with the scrub brush and the baking soda. The oxalic acid will loosen and remove any remaining rust.
Step 6: Rinse the Stones Well
Rinse the stone patio thoroughly with a garden hose. Use lots of water so that the acid is completely diluted and removed. The baking soda helps neutralize it as well.