I don't know the answer to your question, but I do know that mastic should not be used for seaming the joints, and should not be used at all in the shower, and probably should not be used anywhere inside a bathroom. Ever.

If you merely misspoke using the term "mastic" for "the stuff used to stick tile to the wall" and he used thinset mortar, you are fine. However given that you are right about needing to waterproof the Durock in the shower area, and your husband was wrong, I think it is likely he used actual mastic (pre-mixed, from a bucket, labeled either "mastic" or "premixed thinset") and likely intends to use it to install the tile in the shower, and that it wrong, wrong wrong.

I'd wait for others who know more to weigh in about how big a problem you are facing. On the plus side since mastic can be softened with water, even after it has "set" you show be able to remove most of the mastic that has already been applied. But I don't know whether merely removing most of it would be enough. My guess is removing "most" of it, redoing the joints with thinset, and then covering that with a correctly applied liquid waterproof membrane, followed by installing the tile with thinset you'll be fine.

Hi. Mapei Technical services are 1-800-992-6273 (US) and 1-800-361-9309 (Canada). I would give them a call and tell them exactly what product/Mastic was used. When I looked on the spec sheet it seems like it may be OK. On their spec sheet, it says fill with an appropriate filler material. ??? That's helpful! LOL

We used Aquadefense, we used a brush to apply and found it fairly easy.

P.S. You are right, he is wrong! :>)

Here is a link that might be useful: Spec sheet for aquadefense

Hi Bob, my husband use Acrylpro ceramic tile adhesive that comes in a bucket. My understanding is if i comes in a bucket it is mastic and in powder form it is thin set.

Your understanding is correct. While the sales guy at the big box store (or even the writing on the bucket) might claim that it is suitable for floors or for large tiles or natural stone tile or for wet areas, its not. The tile pros here and in the John Bridge forum state that the only place it is suitable for use is on walls, in rooms other than bathrooms, and even then only with small enough tiles that aren't natural stone.

My understanding of the problem with mastic are that it sets up through the moisture in it evaporating. If the tile is too large, the mastic in the center of the tile will never fully set. On floors that means the tiles will shift slightly due to pressure walking, and in a shower as the moisture seeps through the grout and tile the mastic will be continually re-moistened, and will basically serve as a growth medium for molds. If the tile is natural stone, I believe that the mastic can discolor the tile.

I don't know what problems may occur from using mastic on the joints beneath the Aquadefense membrane. I hope one of the tile pros will weigh in regarding that, but I can definitely say don't use any more mastic, anywhere in the shower area.

Hi, Joe at Modern Room Remodels member of the National Tile Contractors Association. Each manufacturer recommends proper installations per ANSI 118.4 In addition, Durock have there own manufacturer recommendations to handle tape seaming joints with a thinset and Alkaline Resistant Mesh tape. Using mastic isn't sufficient to properly bond & firm those joints. In addition AquaDefense states to utilize, for tile and stone installations, use a MAPEI polymer-modified mortar meeting ANSI A118.4 or ANSI A118.11, or an epoxy mortar meeting ANSI A118.15 ANSI A118.3