When doing glass block window repairs you'll want to keep in mind that blocks may have to be replaced during the process, depending on the severity of the situation. These blocks are generally mortared into place, and to get them out of their mounting, will usually involve having to break them. However, simple issues with mortar can be done easily and without causing damages to the glass. You can apply any number of styles to the actual joint, once the grout has dried and it's ready to be molded to your desired shape.
Step 1 - Clean Up the Joints
When repairing glass block window mortar, the first step is to try and clean up the actual joint. This is the area where the blocks come together or even join to your foundation. Take a chisel and a rubber mallet to clear any broken or cracked mortar from the joints to prepare it for the needed repairs. Use a brush to clean the crumbs away from the work area, and be careful not to chisel toward the glass. When chiseling, point the tool away from the glass section, toward the wall foundation to prevent cracking your blocks.
Step 2 - Apply Your New Grout Compound
Mix up your grout and start applying it to the joint in your glass block window. Make sure it is pushed into the recess of the joint completely and evenly to avoid bubbling and air pockets from developing behind your work. You can use your finger or exert force from pushing grout into the joint with your trowel to make an even seal. Do not over-bond the joint, and be sure that the grout is mixed at a proper consistency for the job at hand. It should be soft to the touch very much like sculptors clay after it has been worked and softened by hand.
Step 3 - Trim Up the Joints
Once the grout is packed into the joints of your glass block window sections, scrape any excess materials from the surface. Let the joint stand, and dry until the surface leaves an imprint to the touch of your finger. You can then use your edging tool to shape the grout in the joint to the style you would like, such as a V joint or cross pattern. There are many tiny details you can add using your edging tool, these are the patterns you see when you look into the joint itself, if it indents to a sharp "V" or if it is rounded.
Step 4 - Clean Up the Joint
Once you are done with sealing the joints on the glass block window, you can then use a damp rag and clean it up of any residual crumbs or flakes. You can smooth the joint using the moist rag, and even help get the bond level and even before it dries. If during the process, you manage to break a block, do not try to repair it, simply remove it and replace it with a new one. Trying to seal a cracked or broken glass block will only lead to air and water leakage issues down the road.