Flagstone is popular with homeowners for outdoor uses such as walkways and patios because of its beauty and durability. Over time, the surface of the flagstone will need cleaning to maintain its attractiveness. It's relatively straightforward to clean your flagstone and return it to its original state. Start cleaning you stone with methods that involve no chemicals or cleaners and work your way up in the intensity of cleaner, if necessary.
Use a high pressure nozzle on your garden hose and wash away all the loose dirt and debris.
Fill a bucket with a mix of mild dish washing detergent and warm water. Then, use a scrubbing brush to remove any stains.
Clean organic stains from leaves, sap, and other natural causes with organic cleaners. These cleaners are specifically designed for organic stains and are usually not effective on other types of stains (see Resources).
Use a cleaning product that is specially designed for the flagstone. You can buy these products at your local home improvement store (see Resources).
Scrub the flagstone with a solution of 1 qt. muriatic acid and 2 gallons of water if all of your other attempts at cleaning have failed, and you have safely tested the solution on the stone. Take safety precautions when using muriatic acid. Wear safety goggles and gloves, and do not allow it to touch your skin.
Test any cleaner you are thinking of using on a small, unnoticeable piece of the flagstone.
Be careful with the use of acidic cleaners on stone. They can etch the stone and also remove the polish from alkaline stones such as marble, travertine and sandstone.