Q: The grout on my 2-year-old bathroom tile is giving off a white, powdery, film-like substance. I let the shower dry for a number of days and then used sealant on the grout, but the problem persists.

Any idea of what is going on and what I should do about it?

A: Unfortunately, we have a pretty good idea of what’s going on. You can apply all the sealer in the world, but it won’t help. The problem lies with the grout itself. The grout was either the wrong type or it was improperly mixed when it was applied, or both. One thing is certain, the grout’s failing now, and the only cure is to remove and replace it.

Don’t panic, it’s not as big a job as it sounds — but it is work.

Grout lines measuring more than 1/8 inch wide require sanded grout. Sand in grout functions the same way that stone aggregate functions in concrete. It provides structure for the cement to bond with and makes the mix stronger. Wide grout lines that do not contain sand lack the necessary structural integrity and are subject to failure.

Improperly mixed grout can also fail. It might be too thin, too thick or reconstituted.

Grout that is mixed too thick may not reach from the surface to the substrate when applied. The grout appears solid on the surface but there are voids underneath. Grout that is insufficiently packed into joints can crack and flake.

Too much water in the initial mix can also lead to cracking. The liquid in the grout will eventually evaporate. This evaporation can cause pin-holing in the grout and weaken its structure. The film you describe could be the result of the weak grout breaking down.

Reconstituted grout occurs during the grouting process when water is added to the mix after the grout begins curing in the bucket. Rather than throw away the grout that has started to set, the lazy grouter just adds a little water so that the grout can be worked into the joints. Although the grout makes it into the joints, it will not cure into a hard block. Instead, it will be crumbly and weak. Based on your description of a “white powdery film” on the tile, this is the most likely source of your problem.

Removing and replacing the grout will take a little time, but can be done in stages. Once all the grout is out, replacement goes quickly.

Use a grout saw to remove the existing grout. A grout saw is a handheld specialty tool with a narrow, diamond-impregnated blade that cuts through grout. Simply saw away at the grout joints so that you get as much grout out of the joints as you can.

Remove at least two-thirds of the depth of the grout. Because the grout is failing, it should come out fairly easily. It’s important to remove as much grout as possible so the new grout you install has good purchase on the tile edges and the substrate.

Once the old grout is out, vacuum up any residual dust. Use a sponge and clean water to give the tile a good rinse.

Mix up a batch of new grout. Make sure to use a grout additive instead of plain water. Mix the grout to the consistency of toothpaste. Use a rubber grout trowel to apply the grout. Work diagonally to the grout lines to ensure an even application. Scrape the excess grout off the tile with the trowel and let the grout dry for half an hour. Use your finger to smooth the joints. Wear thin, latex-rubber gloves to protect your fingers during this process.

Gloves are especially helpful when working with sanded grout. Use a clean sponge dipped in clean water to wash the surface of the tile. It’ll take two or three passes to clean most of the film from the tile. Carefully check all the joints during this process and fill in any voids you might have left. Finally wipe down the tile surface with a clean cotton cloth to remove the final residual film and polish the surface. If you happen to have a piece of burlap around, that works great for this step.

Finally, apply a silicon-based grout sealer according to manufacturer’s instructions.

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