its my experience that 3/16" notch trowels are ok with this(the tighter the better(like my women:grouphug: ) i think thicker notch trowels do not work with this type of mastic.
i used it years ago on walls in DRY areas but never in a shower or wet area..i did a good few rip outs (not my work

) and noticed the mastic being wet and un cured.i would never risk using it.thats only my opinion so carry on regardless.
There are numerous flexible cement based adhesives (i like to call them thinsets:lol: ) on the market.
i use them for all my tile including splashbacks(backsplashes:lol: )
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This adhesive(mastic,organic acrylic) your talking about is in my opinion a liability based on what ive seen and what i have read and discussed with other tilers here in the usa. I ripped out a good few bathrooms and noticed on a few that the mastic hadnt cured,was wet and emulsified and to be honest a nightmare.
I have used it in the past but never in a wet area.Its not worth the risk when you can mix your own dry set mortars.I think the bigger the notch on your trowel the more risk you take in achieving a full cure. Im sure the warranties are dependent on full cure instances only.Some tradesmen want to get in and get out(myself included)but grouting a job that has not fully cured with mastic is a problem in the making.
MY opinion only fellas and i dont want to rock your boat :yawn:
ps i would never use it on any natural stone whatsoever