Regrouting directly over dirty, uncleaned grout is a major mistake that guarantees premature failure of the new material. Grout joints must be completely clean, dry, and free of dust, grease, soap scum, and loose particles so that the fresh grout can form a strong mechanical bond with the surrounding tile and base mortar. If you apply new grout over contaminated joints, the underlying dirt acts as a barrier, preventing adhesion and causing the fresh grout to crack, crumble, and wash away almost immediately upon exposure to moisture or routine cleaning. Thorough joint scraping and vacuuming are mandatory preparation steps.
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