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Unread 01-13-2017, 11:25 AM   #1
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Kerdi membrane over drywall mud

I am getting ready to install kerdi in my bathtub surround. I have hardibacker all around the tub but the installers used drywall mud to join where the drywall and hardibacker meet and this seam extends into the tub where I want the kerdi to be.
I started to sand it off but is another option to prime the the mudded areas? If so, latex or oil?

Also, I really want to tile to the ceiling (just using 3x6 tiles) and also have mud above the showerhead height. Tile shop told me to just use mastik up there because it will not get very wet and I don't know whether that is all hardibacker or partially drywall at the very top.
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Unread 01-13-2017, 05:18 PM   #2
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1. Best thing to do is remove as much mud as possible. A sanding block, and a wet sponge can get the majority off.
2. While there are some that are technically approved for intermittent wet areas I personally would have it anywhere near a shower. It costs more than thinset, contains organic matter that can grow mold and mildew, and also doesn't jive with schluter products.
Jack of most trades, master of none...
Ryan McKee
McKee Construction & Custom Tile
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Unread 01-13-2017, 05:51 PM   #3
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Drywall installers usually use the premixed drywall mud. If that's the case, fill a spray bottle with plain water and spritz an area very well, let it sit for a few minutes maybe adding some more if you see it drying out, and you'll be able to remove most of it with a putty knife as a paste (no dust!). What's left will usually wash off with a sponge fairly easily. If they used a setting compound they mixed up with water (i.e., dry powder), then spraying it won't work! You'll want an alkali resistant mesh tape for the seams...well, you really don't need that at all if you're putting Kerdi over it! The Kerdi is way stronger than the mesh tape and will perform the same function.

Unless this is a steam shower, which it doesn't appear to be, regular drywall should be fine on the ceiling. YOu may want to add more drywall screws, since installers often don't put in enough. If you have the industry standard amount and spacing of fasteners, drywall will hold tile just fine on the ceiling. If there's a joint and you're not using Kerdi there, I would use the mesh tape on the seams. You do not need to do this prior to tiling, you can do it during. If there's a seam there, and the tapered edges are at the seam, you might want to fill that area with the tape and thinset prior, so that it is level with the rest of the surface. If you're using a large enough tile, it probably wont' make any difference, but otherwise, your tile might follow that dip when setting the tile. THe magic to getting tile to hold overhead is properly embedding them and having the proper consistency of your mortar.
Jim DeBruycker
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.
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