Kerdi and moisture sensitive stone

With marble I usually use white modified thinset and have no problems. Yes, even on Kerdi.
Depending upon the particular onyx you may be able to use the same thing, just make sure you get full coverage so any imperfections in the thinset do not show through since most onyx is translucent.
Each brand of onyx has specific recommendations as to what to use to set it. If it says to use epoxy I would check with Schluter to make sure that won't create problems - I've never done it so I can't say.

I have done a few jobs like the one you are describing. I used the same mortar (unmodified white) for the whole job. no problems

Some marble and some onyx is very moisture sensitive and can curl when applied with water-added-thinsets. Know your product before proceeding.

Not sure if epoxy thinset is compatible with KERDI.

Bud Cline said:

Some marble and some onyx is very moisture sensitive and can curl when applied with water-added-thinsets. Know your product before proceeding.

Not sure if epoxy thinset is compatible with KERDI.

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I have sent an email to my schluter rep to ask the same question.

I will let you know what he says

Thanks for the info. I actually called schluter the other day and was given a answer that basically left me in the dark. He did mention epoxy but said I needed to contact the stone manufacturer. Im curious to hear what your rep is going to say. The tile store that I get the kerdi from said to use epoxy.

The stone definitely determines what mortar to use. However, if Schluter doesn't want epoxy over Kerdi, you're in a pickle. I would only trust what Schluter says, not the tile store. Those are the same people that sold one of my customers polished glass mosaic and then sanded grout

I know what you mean Angus. Im going to wait and see what the response is from the Schluter rep.

my schluter rep says yes to using epoxy on kerdi, as well as ditra.

One thing I've done when I was using serpentine marble as an accent was to coat the backs of the marble with the epoxy, let it set overnight, and then set it along with the rest of whatever was being installed, with regular thinset. Then the epoxy coating on the back of the stone doesn't allow the moisture from the thinset to creep into the stone, you can set everything with the same thinset (and not spend a fortune setting EVERYTHING with epoxy), and in this case, not worry about compatibility of products. Now, of course, in this case, it's all dependent on how big the individual pieces are. I don't know that I'd want to spend the next week and a half coating little 1/2" pieces of stone for 10-12 linear feet!

Thanks for checking w/ Schluter, I really appriciate it. The guy I spoke with didnt really want to commit to a response.(At least that was my take). Bill, Ive heard that before, and now that gives me another option.
I think i will go with the epoxy if I cant first talk the customer out of using the stone she wants to use. Thanks for all the input guys, Im feeling alot more confident about this one...

Good luck with it.

Bill_Vincent said:

was to coat the backs of the marble with the epoxy, let it set overnight, and then set it along with the rest of whatever was being installed, with regular thinset.

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exactly the way I did it with a few marble tile on Kerdi