Applying a fresh layer of new grout directly over old, existing grout without proper mechanical preparation is strongly discouraged by professional tile installers because the new material cannot form a permanent, reliable bond. Over time, old grout accumulates microscopic layers of grease, soap scum, dirt, and moisture that prevent new grout from adhering securely, resulting in rapid flaking, crumbling, and cracking. If you need to refresh discolored or damaged grout, the correct procedure requires using a manual or oscillating grout saw to grind out the top layer of old grout to a minimum depth of one-eighth of an inch, cleaning the joint thoroughly, and then applying fresh grout.
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