How to Seal Stone Masonry Fireplaces

Place a drop cloth on the floor at the base of the fireplace to catch any spills. Cover adjacent surfaces with plastic sheeting, and then secure the plastic in place with a strip of masking tape along each edge to prevent sealer overspray from getting on them.

Add 1 tsp. of mild liquid dish soap to 1 gallon of water in a bucket and mix thoroughly. Clean the stones on the fireplace with the mixture, using a stiff-bristle brush to remove any accumulated dirt or debris. Rinse off the stones with clean water, and then allow them to dry for six to 12 hours before you continue.

Open the windows to the room to increase ventilation, and put on a face mask to avoid breathing sealant fumes.

Fill a hand-held pump sprayer with a stone sealant formulated for use with your fireplace's particular stone type. Pump the handle of the sprayer up and down a few times to build up the air pressure.

Hold the nozzle of the sprayer 4 inches from the surface of the stone at a top corner of the fireplace. With the nozzle pointed directly at the stone surface, pull the trigger of the sprayer to begin spraying the sealer onto the stones. Use a back-and-forth motion to apply the sealer in rows across the stone. Overlap each row slightly to avoid leaving gaps in the sealer coverage as you work your way down the fireplace surface. Apply a moderate layer of the sealer, wetting the stone down but not allowing the sealer to pool in any one spot or run down the stone face.

Allow the sealer to dry on the stone for about five minutes, and then spray the surface again to provide enough sealer to penetrate the stone surface. Wait 30 minutes after the second application, and then wipe any excess sealer from the stone, using a lint-free cloth. Allow the sealant to dry completely according to the manufacturer's suggested drying time before you use the fireplace.