Can I mortar over the grout I installed in between my 3/8 spaces brick veneers? Given there is enough space in depth, would it stick and look ok? I bought and installed a natural gray grout but it came out darker than expected. Luckily the spaces were deep and I still have a little room to play with. Or would it be better to either, remove grout, which I don't want to, or re apply lighter grout?
Mortar and grout both contain cement so they should form a positive mechanical bond. What may prevent this is if a sealer has been applied to the grout. The depth of the joint is important. If the joint is 3/8 inch in height it should be close to 3/8 inch in depth. This also ensures that enough mortar can be troweled into the joint to bond with the brick and grout. The fastest way to open the joint is with a masonry blade and a grinder. It is a noisy and dusty job so proper safety protection should be taken. Vacuum the joints and dampen the joint and brick. Dampening is key to point work. It allows the water in the mortar to remain there rather than being absorbed into the dry grout and will form a stronger joint. It also makes the inevitable mortar splotches and smears easier to remove from the brick face for the same reason (the water isn't absorbed as easily if the brick is already wet). Mix the mortar in batches that can be used within 30-40 minutes if temperatures are above 80 degrees. If you add a splash of admixture to the mortar while mixing it will be easier to trowel and become more resilient when cured. Use a mortar bag and joint tool if you can. The joint tool is key to forming a professional looking concave (or convex) shaped joint. After mortar has set for a few hours mist with water to slow cure time and increase it's strength. It will lighten significantly once fully dried.