How to Seal a Chimney Indoors

Clear the mantel of any contents, if applicable, before covering it and the floor under the chimney with plastic tarp. Slip on a pair of rubber gloves, safety goggles and a respirator and create a mixture of 1 tablespoon TSP, or tri-sodium phosphate, 1 tablespoon liquid dish soap and 1/2 gallon warm water.

Dampen a plastic scrub brush with the TSP mixture, which is a mild degreaser available at hardware and home improvement stores. Scrub the chimney’s surface with the TSP mixture to remove any soot, debris, grime or dust. Wet a kitchen sponge with plain water and rinse off the TSP mixture. Allow the chimney to dry completely.

Apply a masonry sealant to the bricks with a paintbrush. The sealant protects the bricks and mortar from failing further. Allow the first sealant layer to dry completely, or at least eight hours, before applying any additional layers, according to the package directions.