Removing old, deteriorating grout is strictly necessary before applying fresh grout if you want the new material to bond securely and last for years. Applying new grout directly over crumbling, dirty, or grease-stained old grout prevents the fresh mixture from forming a proper mechanical bond, leading to rapid flaking, cracking, and detachment within weeks. To ensure professional results, you should use a manual grout saw or an oscillating multi-tool to grind out the old grout to a minimum depth of one-eighth of an inch, clean out all residual dust thoroughly, and then apply your fresh grout mixture into the prepared joints.
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