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For a tiling job, I used this charcoal grout:

enter image description here

After a few days, as indicated by the instructions, I applied this sealer:

enter image description here

A few minutes after applying the sealant, these white stains appeared randomly in the grout:

enter image description here

It's been a few days and the stains are still there. What should I do about them? They tend to vanish if I scrub the grout, but I'm not sure that's what I'm supposed to do.

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    I would contact the "help line & product support" for the sealer product. 800-272-8786 or use their web contact form to inquire: custombuildingproducts.com/contact-us
    – Ecnerwal
    Dec 14, 2021 at 14:12
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    @Ecnerwal: good idea, thanks. I have just sent them the question. Dec 14, 2021 at 14:28
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    @Ecnerwal: and I got a reply within 15 minutes. I have posted it as an answer. Dec 14, 2021 at 14:45
  • Efflorescence on #60 can be 'cured' with a #1 sharpie. That it's only on the horizontal lines is a hint it isn't though.
    – Mazura
    Dec 15, 2021 at 18:43

2 Answers 2

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As suggested by @Ecnerwal, I wrote to Custom Building Products' customer support. Got a reply within 15 minutes:

The isolated areas look to be a bit of sealer residue that might not have been wiped or rubbed dry with dry paper towels after each application within 3 minutes of each application?

If so, this can be safely scrubbed off at any time, using a MIXTURE of baking soda, and water into a paste, and using a toothbrush, the white surface residue can be scrubbed off without damaging the sealer protection or original grout color.

There is no need to re-seal the grout.

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    "within 3 minutes" ? pffft. Rag in one hand; paint brush in the other. Apply illiberally; wipe up immediately.
    – Mazura
    Dec 15, 2021 at 3:03
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    I always use a moderately stiff bristle brush to applicate, to work the surface better, so that anything that's going to come off does and gets sealed under it. I'd like to think another shot of sealer, scrubbed the way I do it, would suffice. A regular toothbrush is a little too stiff though, and supple ones are too soft. Use a small parts cleaning brush and be mean to it. - Spray bottles is for tile or stone, not grout : pour some in a can, use a brush and then a rag.
    – Mazura
    Dec 15, 2021 at 19:00
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Did you use all the grout at once? Some grouts aren't dry mixed well at at the factory. If you read the fine print it will say to mix the dry grout BEFORE adding water to.

So if you do it in batches and didn't mix it, you'll get uneven results. When you wash it, it's just getting wet and hiding the problem.

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    I mixed all the grout, for the time said in the instructions, and applied it all at once. There were no stains for four days, and it looked perfectly even. Stains appeared a couple minutes after I applied the sealer Dec 14, 2021 at 12:58

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