How to Seal Mortar Around a Glass Block Window

C.L. Rease

Stains and mold are common on the porous mortar that holds glass blocks in place. Over time, the staining becomes unsightly and ruins the look of the glass block wall or window. Periodically applying a sealant to the mortar fills voids in the material to combat the mortar’s porous nature.

Sealed glass block mortar remains stain-free after installation.

Proper application is necessary to ensure the glass block structure remains stain-free. Even a small gap in the sealant will allow fluids to seep into the mortar, defeating the barrier provided by the mortar sealant.


Keep a running fan in the area to keep sealant fumes from building around the glass block window.

  1. Pour liquid mortar sealant slowly in a spray bottle. Do not allow air bubbles or foam to accumulate inside the sealant.

  2. Move to the highest horizontal mortar line of the glass block window. Hold the tip of the spray bottle nozzle 1/2 inch away from the mortar joint. Spray sealant on the joint as you slowly move the bottle along the seam. Spray the remaining horizontal seams. Spray a light coat of sealant on all vertical mortar seams.

  3. Rub the sealant into the mortar joints using a lint-free rag. Wipe the surface thoroughly until all excess sealant has been removed from the surface of the glass blocks. Allow the sealant to dry for 45 to 60 minutes.

  4. Buff sealant residue from the glass blocks, by wiping across them with a clean lint-free rag. Move the rag in a circular motion if the hazy residue is difficult to remove.