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Grout darkening

I tiled my tub surround last fall. A couple of weeks ago, I noticed one angle at the tub joint is darkening. A tile guy said the sealant might have failed and suggested I let it dry out for several days, then re-seal. Two weeks later, it's still dark (I haven't re-sealed yet). It seems to be embedded in the grout itself, the grout is solid and it won't scrub off. If the problem was mold or mildew it seems like the caulk would be darkening or damaged as well.

While I was working on this problem, I noticed some shadows in the middle of other grout lines. After some research, I think this is efflorescence, or the sealant is failing and it’s taking in dirt. Again, it doesn’t scrub clean.

Any suggestions?

Re: Grout darkening

how was the wall build under the tile?? membranes,bonding agent?

Re: Grout darkening

I used an underlayment, then cement board (not greenboard). It's on an exterior wall, not a plumbing wall, so it can't be a leak. It is in the area that gets the most water when the shower is running, but it seems like other joints and lines would be deteriorating, too.
When you say bonding agent are you asking about the adhesive? I used a Henry mastic for showers.

Re: Grout darkening

Have you tried a solution of Clorex to bleach it out? How about Tilex or CLR? Both have bleaching capabilities. If it works, then reseal it with a good grout sealer.

I do believe that the sealer is supposed to bead up if it is working properly when water is applied over it.

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