Brick is very porous and for that reason, most sealers have trouble bonding to it. The best type of sealer to use on brick is a silane/siloxane water repellent. Unlike other sealers, a silane/siloxane is not designed to fill in the pores. Instead, it lines the pores and the surface with an invisible hydrophobic barrier that keeps water and other liquids on the surface. Water will “bead” off the surface coating.
Silicates can be used to strengthen the brick, but not waterproof it. Acrylics should not be used on brick at all because bricks are too porous for the acrylic to develop a bond.
Silane/Siloxane Water Repellent Manufacturers:
I have a brick and mortar retaining wall with dirt behind the wall. The wall is about 3 feet high. When it rains, the back of the brick will always be wet during the rainy season.
If I put this sealer on the front of the brick, will I still get efflorescence even though the back of the brick gets wet during the rainy season?
Sealing at the source is the only true way to stop efflorescence. Moisture and water carries salts through the brick, causing efflorescence to form on the other side. If you stop water penetration at the source, efflorescence won't form on the opposite side. If your only option is to seal the front side of the brick you can use a Densifier first (such as the Armor S2000 or Armor L3000) followed by a water repellent sealer like the Armor SX5000. A Densifier will form a calcium structure within the pores, reducing the pore size and the water repellent will reduce the movement of moisture.