Attempting to apply fresh grout directly over existing old grout without grinding out the joints first is a shortcut that inevitably leads to failure. Because old grout is porous and contaminated with dirt, grease, soap residue, and cleaning chemical films, a new layer of grout cannot form a permanent mechanical bond with the old material. Within a short time, the thin veneer of new grout will crack, flake, and detach completely from the joints. Proper installation standards dictate that you must remove the deteriorated top layer of old grout using a grout saw or multi-tool to create adequate depth for the new grout to lock securely into place.
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