Kerdi and drywall mud
Ideally, your shower area is virgin drywall. But, what do you do if the builders mudded the whole area? Schluter says prime it with a laytex primer, but doesn't give any recommendations. What should one use that has been tested and reliable? The danger of going directly over the mud is it could soften and you might end up with a debonded area. personally, I don't think it would be a big deal as it would reharden, and probably then never get wet again, but the what if scenario could come into play, especially if a large area was skimmed out with the non-hardening stuff.
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters),
Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.