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I am remodeling a bathroom that was originally done with 1/2 inch drywall. I opened-up the walls to move plumbing and electrical as required and then covered-up those sections with new 1/2 inch drywall.

The shower and tub areas are going to have tile on the walls as well as the entire bathroom floor. I've already done Ditra + tile in other rooms so no questions there.

Here's my problem: Whoever built this room put on a skim coat that is, by the looks of it, about 1/8 of an inch thick in some areas. So my new drywall is 1/8 thinner than the existing wall in some sections and a perfect match in others.

Now I have the problem of matching all of this up before installing the Kerdi membrane.

What type of mud do I use for this? Is a general purpose compound good? Remember that I will have to apply unmodified thinset over it in certain areas in order to adhere the Kerdi membrane.

I don't think I can skim coat with thinset. I'm not good enough to achieve a smooth finish and it can't be sanded. My best guess is that the solution is to skim coat the entire room, tape corners as needed and then thinset where the Kerdi membrane is applied.

Is a 1/8 skim coat strong enough to support the tiles? I have no clue how well these layers will bond to each other.

Can I use a premixed mud compound or do I have to use a setting-type material?

Just to make sure I have the process straight: First you skim coat the entire bathroom and then you go back and tape conventionally. Correct? Or is it the other way around?

I've done a bunch of tape-and-mud-ing and even fixing very large (2 foot) holes in other rooms. I am generally comfortable with that aspect. I just haven't skim-coated an entire room and much less applied tile on top of it.

Thick mud layer

Shower corner

Tub corner

Vanity corner

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  • So just to be clear, you want to use mud to level the wall, then kerdi, then thinset. Correct?
    – Machavity
    May 1, 2019 at 1:19
  • I have to even out the walls with something. I am thinking that would be mud. I then have to adhere the Kerdi membrane to the walls. That would, per Schluter, require unmodified thinset. After that the tiles would go on the Kerdi membrane with, again, unmodified thinset. I want to make sure I use the correct type of mud (or whatever) to level the walls and still have structural integrity/strength for the tile installation.
    – martin's
    May 1, 2019 at 4:11
  • Have you reached out to Schluter and asked? I bet they can give you specifics on what's acceptable under the membrane, both from a materials standpoint and in terms of how level it has to be.
    – dwizum
    May 1, 2019 at 16:42
  • I spoke to my local Schulter rep. He said to skim coat normally and then make sure everything gets a coat of paint primer before thinset+Kerdi. Other than that, no special recommendations. I was looking for a specific recommendation of the kind of mud to use. There are probably some 15 different types at H.D., each with its own properties. I am getting the sense that a good general purpose mud will be more than sufficient.
    – martin's
    May 1, 2019 at 20:30

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I don't think you'll need anything other than regular joint compound here. I would make sure you buy the widest trowel you can afford and simply work as much compound as possible to level the wall. Your kerdi board placed over the top (when used with the proper thinset) will block the water from reaching this layer.

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