There should be no reason to use epoxy in your tile work. You may want to post pictures of this so we can see what you see.
But the two surfaces are on different planes and can move differently. Does epoxy grout have sufficient elasticity for the outside corner ?I'd find a good epoxy grout that is suitable for narrow joints.
Thanks!!There is no need for anything epoxy in there.
Out of curiosity, it looks like you're mixing Kerdi and RedGard - why is that?
That is a standard corner that should be grouted with the standard grout that's used in the rest of the shower. The concept of caulking rather than grouting changes in plane really doesn't apply to situations like that. Furthermore, Kerdi showers, i.e. ones built with actual Kerdi Board as yours appears to be, normally doesn't need this precaution anyway because of the buffering movement inherent in the Kerdi foam itself. With hard cement board you have somewhat of a different situation. There is some uncoupling inherent with the Kerdi Board (and other Kerdi foam materials), just like there is with Ditra.
Yeah, never used epoxy for anything in my life.You may not like epoxy grout. Think "bondo", which is epoxy. I am not sure that is a good correlation, but I don't think you will benefit from straying from normal grout application even in this area since you used Kerdi.
I would say so, but I still wouldn't bother with epoxy grout. However if you wanted to shape and polish something, then yeah I'd go with epoxy. I do understand your concern, as cleaning out too much grout on mitered tile corners has been a problem for me in the past. You could try taping the edges so there's not much grout to clean off, and shaping the grout with your fingers. Use a fairly stiff mix, don't get it to fluid. Then lightly smooth it out with a wet finger or softly with a sponge.If I DO choose epoxy, epoxy grout is recommended over a general stone epoxy adhesive.