The best type of concrete sealer to use on brick chimneys is a Silane-Siloxane water repellent sealer. Silane-Siloxane water repellent sealers will  help to reduce staining and deterioration caused by water absorption. They will also Brick Chimney Sealers help to reduce the formation of mold, mildew, and efflorescence. Silane-Siloxane water repellents can be used to seal the entire brick chimney, grout included.

Siliconate water repellent sealers and silicate concrete sealers are two other types of penetrating sealers, but they aren't the best type of sealer to use on a brick chimney. Siliconate water repellents can leave behind a white residue when applied to brick, and silicate sealers will densify, but they won't repel water. If over applied, silicate sealers can also leave behind white surface crystals. Silane-Siloxane sealers are the best type of sealer to use on brick chimneys.

In most cases, a brick chimney sealer is all that your chimney needs, but if your chimney has issues with deterioration, cracking, moving, or leaking, you may need more than just a chimney sealer. A chimney is made up of a few different parts, and each part should be analyzed:

  • Flashing: The flashing can be found at the bottom of the chimney where the chimney and the roof leak. This is the number one source of water penetration. Make sure the flashing is in good condition, properly installed, and properly working.
  • Chimney Crown: Chimney crowns are usually made of a hard material, like metal or concrete. Make sure the chimney crown is in good working condition. If the crown is made of concrete, apply a Silane-Siloxane water repellent sealer to that as well.
  • Brick and Grout: At some point, a chimney will need to be repaired or repointed. If you have issues with missing grout lines, or the chimney isn't straight, consider repairing the grout and repointing the chimney as structure will always be very important. 
  • Chimney Cap: The cap goes over the opening of the chimney. It helps to reduce water and snow from going down your chimney into your fire place or furnace. If you don't have a cap, you risk your fireplace or furnace filling up with water, and you risk having your chimney deteriorate from the inside. A cap is never a bad a idea.