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Before you start, yes, I did know that grout would be hard to remove from the slate. So did the guy doing the job, as I made sure that we discussed it before hand. I was of the impression that he was going to grout carefully, more like pointing. We went away for the weekend while he finished the job, and he did no such thing.

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One corner of the room looks ok, so I guess that'd be the corner that he started in, and then he got impatient and rushed the rest of it, and left us with this mess.

He did put a coat of sealant on before grouting, but given how hard it appears to be to remove, I'd say that's not helped very much.

What's the best course of action? I have some grout stain remover, which says it's 10% hydrochloric acid. If I use it neat and scrub like hell (with a standard washing up scourer sponge) I can make an improvement. A very slow, tedious, improvement.

I'm guessing this is the best I can hope for, but maybe I could be using a stronger acid? What's the best scrubbing implement? Could I be using some drill-attached agitator of some sort?

Any tips hugely appreciated, thank you!

Specifics:

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    Would get a whip and get the guy back to finish the job.
    – crip659
    Commented Jul 13, 2021 at 10:32
  • Agreed, but I'm hoping to come up with solutions to help him finish the job well and this year. Sadly he's a neighbour who's semi-retired from his flooring job (which I now know was 90% wood, 9% carpet, 1% stone) so I don't want to fall out with him.
    – Codemonkey
    Commented Jul 13, 2021 at 10:36
  • Don't know enough about slate, but if using power tool would not use much stiffer than a buffer wheel or soft rotating brush. Would not use stronger acid unless told by manufacturer, they might have better safe ideas.
    – crip659
    Commented Jul 13, 2021 at 10:50
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    Would it be a "fall out" to solicit his expert advice on how you could best clean up the excess grout? I think that presenting the issue but abstaining from blaming this neighbor is a possible route. I am the type of person that would have a hard time ever complimenting his work without explaining the caveats.
    – MonkeyZeus
    Commented Jul 13, 2021 at 13:07

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I had a very similar problem and used a 10% hydrochloric acid stain remover. The nylon souring pads worked slowly but were better after I let the soak in for a few minutes. I finally switched to steel wool but only after experimenting on a spare piece of tile. The steel wool worked great but it was still a slow process. I stayed away from rotary brushes as to not fling the acid around the room.

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