Flagstone pavers are naturally occurring stones that are carved into irregular shapes. They are laid on the ground to create patios and walkways. Since flagstones are porous, they can absorb stains over time or develop a white chalky appearance on the surface that standard cleaning does not remove. In these cases, you must wash the flagstones with a mild acid solution to restore them to an attractive appearance.
Sweep the top of each of the flagstones with a push broom using vigorous force to remove all surface debris from them.
Put on safety glasses and gloves as well as long sleeves and pants to protect yourself from the acid.
Fill a plastic bucket with a mixture of 1 part sulfuric acid to 20 parts water. Sulfuric acid is the same acid that is used to clean swimming pools and is available at hardware stores and pool supply companies.
Pour the acid over the flagstones until the surface of each stone is completely wet.
Allow the acid to sit on the flagstones for two to three minutes to begin dissolving the stains.
Scrub the surface of each flagstone vigorously with a brass, wire brush using circular motions.
Rinse the surface of the flagstones with spray from a garden hose to wash away all of the acid solution from them.