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I am renovating my bathroom and noticed a tile was chipped after it was installed. I carefully chiseled the tile off the wall but a thin layer of cement board came off with it, see pic below. I used 1/2 inch durock and a few coats of aqua defense. The fiberglass on the cement board is still intact. Can I apply a layer of Thinset to repair the cement board, follow up with a few coats of aquadefense? I thought of cutting out the board section and replacing it but I wouldn’t be able to tape the seams, and the picket tiles don’t lend themselves to nice square repair cutouts. Any advice?

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    What specific product are you using for thinset?
    – popham
    Jan 20 at 18:20
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    That cement board looks like it's toast. How much flex does it have if you push around all of that rubble? Is that an interior wall with just drywall on the other side and nothing else (like kitchen cabinets or permanent stuff like that)?
    – popham
    Jan 20 at 18:23
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    The waterproofing may be a problem. Where is this located? Is it part of a bath surround with the bad spot well away from the shower head? Is it part of a shower surround with the bad spot elevated at least 2' off the ground?
    – popham
    Jan 20 at 18:44
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    You might be able to push some epoxy through the mesh, maybe.
    – Huesmann
    Jan 20 at 18:59
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    It you can't do good enough on repairing the waterproofing and/or you're worried about it, then epoxy grout is an option for waterproof grout. My understanding is that it's really easy to screw up, though, so I wouldn't try it in your position. I don't think I would try it myself. I expect that you'll be fine doing what you can with whatever's left of your AquaDefense (assuming that you still have a bit leftover).
    – popham
    Jan 20 at 20:35

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Cut open the wall's surface from the other side and thinset a cement board patch over all of the fractured cement board. The patch should extend a good 6" to 12" over undamaged cement board as long as there aren't studs in the way. For studs in the way, run the patch right up to them. This will restore the stiffness to the damaged cement board.

Cut out the rubble and cut a little patch of replacement cement board for the opening. Install this patch to the big patch while its thinset is still workable. Fill the gaps with thinset (no tape) and do your best to waterproof it again.

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