You do not necessarily have to remove every single trace of old grout down to the bare tile edge, but you must grind out a substantial portion—at least the top one-eighth to one-quarter of an inch—to ensure the new grout can bond securely. Leaving old, dirty, or crumbling grout intact without sufficient depth prevents fresh grout from forming a proper mechanical bond, resulting in rapid flaking, cracking, and detachment. Using a manual grout saw or an oscillating multi-tool to clear out adequate depth and cleaning the joints thoroughly is essential for a long-lasting regrouting project.
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