Crumbling grout poses a problem, especially in the bathroom, since it allows water to seep behind the tile and cause serious damage. It is not a good idea to try and repair crumbling grout or place new grout over a crumbling surface. Instead, the old grout must be completely removed and replaced.
Regardless of the cause of crumbling grout, the old material will need to be removed before any repair work can begin. Use a grout saw to carefully remove the old grout. Work slowly, taking care not to damage the tiles. Use the saw to cut and scrape away the grout down to the wall or subsurface. Do not scratch or gouge the subsurface, but be sure to remove all of the grout.
Crumbling grout could be a signal that water has damaged the surface beneath the tile. In the most inconspicuous location possible, carefully pry away one or two tiles after removing the grout and inspect the wallboard or surface underneath. If there appears to be damage, all of the tiles will need to be removed and the damaged material replaced. Fortunately, subsurface damage isn't the only cause of crumbling grout. Acidic cleansers, improper mixing at the time of installation and regular use over many years can all cause grout to crumble.
If it is clear that there is no damage to the surface underneath the tile, the surface must be cleaned prior to re-grouting. Use a vacuum with a hose attachment to remove any dust or debris left behind during the grout removal process. Prepare the tile and joints for grout application by cleaning the area with a solution of 1/2 c. vinegar mixed with 1/2 c. water. Allow the surfaces to dry thoroughly before applying new grout.
Since grout keeps water from penetrating to the subsurface, it's important to replace the crumbling grout that was removed. Mix the grout according to the manufacturer's instructions, and apply by spreading it across the space between the tiles with a grout spreader. Make sure all of the joints between the tiles are filled. Remove any excess grout by first scraping it off with the grout spreader and then wiping it away with a sponge. Let the grout dry completely, usually about three days, and then apply a sealer according to the manufacturer's directions.
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