Applying new grout directly over old, existing grout without proper preparation is generally not recommended by professional tile setters because the new layer lacks sufficient bonding depth to adhere securely. Over time, old grout accumulates dirt, grease, soap scum, and moisture that prevent new grout from forming a permanent mechanical bond, leading to rapid cracking, crumbling, and flaking. If you wish to refresh stained or damaged grout without a complete tear-out, the proper approach involves using a manual or oscillating grout saw to grind out the top layer of old grout to a shallow depth of at least one-eighth of an inch, cleaning the joint thoroughly, and then applying fresh grout or utilizing a specialized epoxy grout stain.
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