Sealing bricks on a chimney creates a waterproof barrier preventing water from leaking into your home. Some water leaks are ambiguous and many homeowners and professionals cannot locate the entry point. Even without an exact entry point, sealants create a tight barrier to stop the leaking. Water-repellent sealers will help the chimney to shed water rather than absorb. Masonry surfaces, especially mortar joints, benefit from waterproofing.
Inspect the chimney for mortar joint cracks, splits or crumbling. Repair damage prior to sealing. Sealants with not effectively cover large gaps. Chip away damaged mortar with a masonry chisel and hammer. Replace the joint with new mortar.
Clean the bricks and mortar joints with detergent and water. Scrub away surface stains, dirt and debris with a stiff scrub brush. Use plain water to rinse the brick and mortar surfaces well. Allow the masonry to dry completely. Moisture and dampness will affect the level of sealant penetration.
Stir a siloxane-based penetrating sealant with a mixer bit attached to a power drill.
Apply a penetrating sealant by brush, roller or sprayer to the entire surface of the chimney. Allow the penetrating sealer to dry completely, generally six to eight hours. Apply a second coat of penetrating sealer to the bricks and mortar to complete the seal. A third coat is not a requirement, but if you have enough sealant leftover, brush, spray or roll on a third coat after eight to 10 hours because sealants lose effectiveness after opening.
Wear a safety harness, respirator, safety goggles and gloves when using sealants on a chimney. Apply penetrating sealers when temperatures are above 10 degrees Celsius. Reapply siloxane-based penetrating sealant every one to two years.