Natural stone patios are typically constructed of highly durable siliceous stones. A drop of muriatic acid or vinegar in a hidden corner of unsealed stone will fizzle and bubble immediately if the stone is not siliceous. Knowing the difference gives you the opportunity to purchase specific cleaning products. Otherwise, a soft brush, bleach and a substantial amount or elbow grease is necessary to clean the stone patio.
Mix a solution of 1 part chlorine bleach to 1 part water. Fill a spray bottle with the solution. A bleach cleanser, such as a bathroom cleanser, can be substituted, but those typically have a lower ratio of bleach to water.
Spray the stones until they are thoroughly damp. Start in a corner that is out of the way to test the solution.
Scrub with a hard bristled brush until the liquid begins to darken and appear grimy. Rinse thoroughly and inspect the stone. Stop using the hard-bristled brush and switch to a soft-bristled brush if the stone begins to appear gouged or scratched. Complete the remainder of the patio and rinse the structure thoroughly. Allow to dry for 24 to 48 hours.
Spray the surface of the patio with a concrete sealer using a pump sprayer. Hold the nozzle 12 inches from the surface of the stone with the nozzle set to a wide spray. Spray with broad, even strokes until the surface is damp with the sealer.
Brush the sealer into the stone with a wide-headed broom.
If you know the specific type of stone used in the patio, you can buy specialised chemicals for stripping and cleaning the stone. Sealing the stone with a cement sealer will protect it from further instances of mould or mildew and stains. Sealants typically need to be reapplied once a year.