Applying wet grout over dry grout typically arises from the need to replace or repair old grout in between wall or floor tiles. Dirt and mildew can discolour the grout's surface and, over time, make cleaning the grout difficult. Grout shrinkage and cracks are additional problems that require a top-up of wet grout to ensure a successful seal between tiles. Solving your grouting problems requires a preparation procedure in addition to the new grout application.
Clean the tile surface with a dilute detergent to remove dirt and grease, using a lint-free cloth. Use a clean toothbrush to sweep along the grout lines to dislodge any crumbling grout. Inspect the grout lines to determine the scope of your project and whether you need to regrout only a few tiles or the whole tile application.
Remove discoloured and cracked grout to leave a solid base of dry grout. Typically, scrape away 1/16 to 1/8 inch of the grout line depth. Use a manual grout scraper tool for a small project or a powered oscillating tool with a diamond blade, as recommended by the North American Tile Cleaning Organization, if you have a large area of regrouting to prepare.
Use a brush attachment on a vacuum cleaner to remove grout dust from the tiles and work area. Use a ready-mix latex-based grout to regrout over a dry grout base, because the latex additive enables the bond between wet and dry grout.
Apply the grout, using a rubber grout float or squeegee. Press the wet grout over the dry grout in a side-to-side sweeping motion across the grout line. Wipe over the tiles with a barely damp sponge to remove any excess wet grout within 10 minutes of the grout application.
Polish the tiles with a lint-free cloth to remove the fine layer of dilute grout that the sponge deposits. Allow a minimum of two days for the grout to dry, or follow the drying time according to the grout manufacturer's instructions.
Use a utility knife to quickly remove a line of old grout stuck to a tile edge. Apply a grout sealer to the new dry grout to prevent mould penetration.
Wear a respirator when removing dry grout to avoid dust inhalation. Wear rubber gloves to protect your hands from the chemicals in the grout.