Bathroom Remodel - Resurfacing Plaster over Concrete Block
I'm in the middle of a bathroom remodel, which is going quite well. I'm approaching a piece of the remodel which I've been debating for a while and I wanted to get feedback from those who know: what's the best way to attach drywall directly to plaster/concrete wall?
The bathroom consists of 1.33 exterior walls. The .33 is the concrete block wall, of which, 1/3 is exterior and 2/3's is interior. This wall is currently plaster over concrete block, and I've added a layer of thinset in an attempt to even out the wall due to old mastic left over from an unfortunate tile job. The other exterior wall is already firred out and I will simply attach Wedi to the fir strips.
I know the first suggestion will be to fir out the walls. This is not an option due to limited space. The plaster work already prothrudes over the edge of the bathtub, firring would cause the wall to overlap the opening of the tub, replacing the tub is not an option, and neither is moving the tub to another section of the room; it's a fairly confined space with limited options. For the record, the plaster work is in sound condition, no signs of moisture infiltration exists on the concrete block wall.
For the section of the wall which is a part of the tub surround, I spoke with Wedi on how to adhere their board to the concrete block wall. They advised the thinset layer I've added, followed by another layer of thinset applied with a 3/8" trowel, and masonry screws every three square feet. I intend to then tile over the Wedi. This seems to be sound advise, but I'm only using Wedi for the tub surround for its waterproofing capabilities, I don't want to use it for the rest of the wall if it's not necessary.
My initial thought was to use Wedi's advise for the drywall, so I purchased some 1/4" sheets and intended to adhere those with thinset and masonry screws. However, I don't know how well thinset would work as an adhesive for the drywall, and I'm concerned it won't last the long haul.
I suppose one option would be apply all Wedi to the concrete wall, but that gets costly and I wanted to hear from the folks here before proceeding down that path. Are there perhaps other ways of prepping the wall partially for tub surround/tiling and also for typical paint and interior treatments for the rest of the wall that don't require the addition of Wedi or drywall? Maybe drywall compound to smooth out the whole wall, coat the tub surround section with redgard, and just typical finishes for the other sections of the wall? It would be nice to avoid having to drill a lot of holes into the concrete block wall. I appreciate any help and suggestions!