Applying new grout directly over old grout in a shower environment is strongly discouraged by professional tile installers because the new layer will fail to bond securely. In high-moisture shower enclosures, old grout lines accumulate microscopic layers of soap scum, body oils, hard water mineral deposits, and mold spores that prevent fresh grout from adhering properly. This lack of a secure mechanical bond causes the newly applied top layer to crack, flake, and crumble away within a very short period. To achieve lasting results, you must grind out at least the top one-eighth to one-quarter of an inch of the old grout before applying fresh material.
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