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I used Mapei type 1 thinset in a shower area over a waterproof membrane (red guard) . The ceramic tile job is about done. I trusted a friend knew what he was doing that was helping me and after reading the information on the plastic pail it says not to use this. It appears to be adhering just fine. Am I heading for failure down the road?

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From their product information:

• Do not use for exterior applications, steam rooms, shower floors or areas subject to water immersion.
• Do not apply over waterproofing, crack-isolation or uncoupling membranes.

So yes, there are no guarantees the adhesion or underlying waterproofing will last. The issue is interaction between acrylic based adhesive and the acrylic based waterproofing. This adhesive, a wet "mastic" (or "clay"), will take days to dry, and in the meantime can dissolve the waterproofing membrane. I have personally experienced failure with this mastic applied to an acrylic primer/sealer, so although not the same, it is related to the warning and limitations in their product sheet. Also in dried form it does not have the strength provided by thinset, and so it can fail, due to movement, over cracks or and decoupling membranes.

This is a real risk you have to weigh against the work and cost of re-doing the tile job. But right now is the best time to start over.

The solution is to pry off all tiles (quite easily accomplished if tiles are recently applied), and start over with thinset cement. Scrub the substrate clean, scrub the tiles clean, re-seal the substrate, and work with thinset and a 1/4in trowel.

The less than ideal alternative re-work is to grout the tiles with epoxy grout. This will be a bit better than leaving it as-is. Epoxy grout seals better and will help with adhesion of the tile.

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I always use the type 1 over red guard. I know it says not to, but I frankly got tired of researching& seeing contradicting information. In the process of hanging durock now for present bathroom remodel& plan on using the same method I did 5 yrs ago with the other bathroom. I've asked several contractors I trust, but can't afford to hire& was told it would be fine.

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    Do you at least have evidence that this works well in the long term? Advising others to completely ignore manufacturer instructions and invalidating the product warranty without at least anecdotal evidence that this works strikes me as somewhat irresponsible at least.
    – FreeMan
    Commented Apr 20, 2022 at 13:17

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