Sealing natural cast stone helps to maintain the stone’s aesthetic qualities and may stave off problems such as efflorescence, etching from acid rain and UV damage. Exterior sealants also protect the stone against damage caused by water infiltration. Achieving a comprehensive seal of the stone requires appropriate technique and must be done under the right weather conditions.

Get the Stone

Wash the stone, using a garden hose. Remove dirt buildup with a mild detergent such as dish soap. Employ degreaser to get rid of grease stains on cast stone. Remove mortar stains by brushing a solution of 1-part uric acid into 6-parts water on the stain. Don’t allow the acid to get on the stone. Permit the stone to dry completely. Mask any areas that will not be sealed.

Put on the exterior stone sealant, using a paint roller or paintbrush. Brush on the sealant in even coats. Permit the sealant to permeate the stone for a quarter hour.

Wipe off the excess sealant, using a clean rag.

Apply a second coat of the sealer if the organic cast stone is porous. Alternately, determine if the stone wants a second coat by placing a few drops of water on the stone. If the water disappears after about one minute, the stone is still porous and needs another coatnonetheless, if the drops of water are still about the stone a few minutes later, the stone probably doesn’t need a second coat.

Permit the sealer to cure for 24 to 72 hours. Don’t walk on the stone for at least four hours after applying the sealant.

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