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I have removed the previous tile and cement board from the bathroom. I knocked down all the major ridges and bumps from leftover thinset on the subfloor. What remains is a very thin layer on thinset on the subfloor that I can’t chisel out. Is it ok to apply thinset and install new cement board on top of this or do I need to somehow sand it all down until it is bare wood? The floor is level.

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There is no need to remove the existing thinset beyond the rough leveling you have done. In fact, it is desireable to apply a skim coat of thinset on a possibly bumpy wood underlayer first, to make the surface as smooth and void-free as possible. Grinding off a previously applied layer beyond that makes no sense whatever, and will just make things worse.

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Technically yes, you can install on top of the old thinset if it's smooth enough.

But... the new thinset (or any other adhesive) is going to rejuvenate what's left on the floor. It will slip around and resettle with even the slightest bit of weight, causing a floating effect. While it can be done, in my experience it just makes a muddy mess that's hard to work with and prone to failures down the road.

Your best bet, and I know money can be tight around the Holidays, is to set a day aside and go rent you a machine (I think it's called a concrete floor resurfacer) to do the hard work.

There are two main types:

Abrasive Removal - you pour on a mild acid presoak. Then the machine has a big spinning disc on the bottom that eats aways at it slowly. This is similar to buffing a floor, it just takes a bit longer. Generates a lot of dust

Corrosive Chemical Removal - The machine is effectively a pressure washer with a spray shield. Muratic acid (or similar) goes into the machine and it blasts the floor with it and a mixture of water. This is the best option in my opinion. However, the machines that are made to control overspray and drainage for indoor use are about three times the cost of an abrasive machine.

There are hybrids as well, but generally they all fall somewhere on a spectrum between those two methodologies.

If you end up deciding to leave the thinset, I'd recommend using a polymer or latex additive with your adhesive. This will help to fill in any crevices that may form in the adhesive layer after your cement board goes down.

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    Hey Chuck thanks for the feedback. This led me down a rabbit hole until I saw a video on YouTube where a diamond cup wheel on an angle grinder was used to remove the thinset. I’m going to give it a try since I’m only dealing with a small area in the bathroom.
    – Niko
    Commented Dec 24, 2023 at 2:10
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    What do you mean by a "slip around and resettle"? The cement board slips around before you can fasten it with alkali resistant screws? Is such slipping around the "floating effect"?
    – popham
    Commented Dec 24, 2023 at 3:19
  • Or perhaps you could use something like Ditra instead of cement board, and not even worry about it.
    – Huesmann
    Commented Dec 24, 2023 at 13:57
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    @popham It's a bit hard to explain. It follows something along the lines of: - Everything seems normal when board is screwed down. - Some time later (< 2 weeks) the old adhesive revives into a non-Newtonian emulsion. - Weight from above (use) revives more adhesive causing expansion. - pressure builds up over time creating pockets of liquified adhesive mud. - regular use eventually leads those pockets to seams and gaps. - Failure point. It can be cracks, leaks, busted tile, etc. I've only ever encountered it twice, so I can't speak to the contributing factors beyond excess adhesive
    – Chuck Terry
    Commented Dec 25, 2023 at 19:47
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    OP is talking about cementitious thinset, a product which does not "rejuvenate", does not "liquify", does not "revive", and most certainly will not allow a properly fastened sheet of cement board to "slip around" or "settle".
    – kreemoweet
    Commented Jan 23 at 17:38

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