You do not necessarily have to remove every single trace of old grout down to the bare tile base, but you must grind out a substantial portion—typically at least the top one-eighth to one-quarter of an inch—to ensure the new grout can form a secure mechanical bond. Leaving old, contaminated, or crumbling grout intact without sufficient depth prevents fresh grout from adhering properly, leading to rapid flaking, cracking, and detachment within weeks. Using a manual grout saw or a power multi-tool to clear out adequate depth and thoroughly vacuuming the joints is an essential preparation step for a successful regrouting project.
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