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Old 10-14-2017, 06:32 PM   #1
makethatkerdistick
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Wolfgang's Bathroom Remodel

Hello there,
I just got finished pulling off $150 worth of Kerdi from my shower walls. I discovered what seemed 80% coverage with the rest being tiger striped and occasionally no contact at all. I was afraid that this would result in an inferior tile installation.

I followed the Schluter instructions religiously: Kerabond mixed to a fluid consistency but still able to hold a notch, pre-wetted the Hardie board multiple times, worked the Kerdi into the wall from the center out with a drywall taping knife. I even used the Kerdi trowel for combing the mortar.

I wetted the Hardie backer a lot. It kept absorbing moisture readily. And yet at some point during the installation process the ridges must have become too rigid to be worked into the Kerdi. I consistently pre-wetted sections for 15 minutes continuously. Was that perhaps not enough?

After one first frustrating attempt, I even skim coated one section with Kerabond and let it set before I applied a regular layer of thinset to work the Kerdi into the Hardie's surface. That resulted in improved bonding but still not 100% coverage.

I am beyond frustrated but I do want to learn to do it right. I want to stick with Schluter as I already have the drain assembly and as I love the liner-free base design. I plan to make a mud base but for now I need to get the walls right first.

Here are the questions I have:
1.) Can I seal the (now mortar coated) Hardie backers to reduce water absorption? If yes, what sealant will ensure proper bonding?

2.) If I used Versabond instead, would that be an improvement in achieving coverage on Hardie backers? In that case, could I forgo the sealing?

3.) I have to eliminate some ridges in the mortar first. Should I sand it down and the give it a skim coat to get it even? What device/abrasive do you recommend?

4.) Does "backbuttering" ever so slightly the Kerdi increase my chances of 100% contact? It sounds awkward but I am willing to do it to get the perfect bond?
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Old 10-14-2017, 08:43 PM   #2
workhurts
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Sorry, can't help but I've read quite a few threads on people with issues doing the hardi/Kerdi combo. I'm sure someone will be here to help.
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Old 10-14-2017, 08:51 PM   #3
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1. You could try using something like mapei primer T. Might be something to check with them about though.

2. Doubt it would make much of a difference

3. Rub brick?

4. I would say that would be a recipe for a bigger mess than what you think
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Old 10-14-2017, 08:53 PM   #4
jadnashua
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Hardiebacker is particularly thirsty...some people go to the extreme of using a garden sprayer to wet it down really well. It's not too wet unless there's liquid water beading up on it, and even then, a few minutes wait, and it's good to go.

Plain drywall is easier, but for those that insist, it does work on other materials, but you have to get things damp enough so that it doesn't suck all of the water out of the mortar before you can try to embed the membrane. The thinset must still be fluid to be embedded properly. On plain drywall, a wipe with a wet sponge, and it usually works fine...HardieBacker needs LOTS more to work. It's best to also use smaller sections of the membrane until you get proficient, that way, you can get it covered before things dry out. Mortar needs to stay damp to hydrate (cure) and gain its design strength...it can't do that if the substrate sucks all of the water out of it.
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Old 10-14-2017, 09:02 PM   #5
makethatkerdistick
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Thanks all for your replies.

So, the Mapei primer might be a choice. Anyone here who's used it to get a grip on the Hardie?

OR just using more water, I take it. I have used a small spray bottle and a sponge and gone up and down in smaller sections for many minutes each. But I suppose that wasn't enough. I certainly didn't just do a quick wipe.

What do you guys think my mistake was?
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Old 10-14-2017, 10:23 PM   #6
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Quote:
Originally Posted by Wolfgang
What do you guys think my mistake was?
Either consistency of the thinset mortar, or the lack of moisture on the Hardibacker, or some combination of the two.

Had you come here before you started, I would have strongly discouraged the use of Hardibacker under Kerdi. It's very difficult to work with when installing Kerdi.

You're going to have to soak it pretty good immediately before putting thinset up, then work quickly to get it spread. You really don't have but a few minutes to get it spread out and covered up before it starts drying. Don't have air blowing in the room, as it will contribute to drying too quickly. For a six-foot long section, I would try to go from wetting the board to having it covered in five minutes or so, or else you'll keep having the same problems.

Once it's covered, get it spread out in 5-10 minutes. You may even have to resort to combing the bottom half of the wall, covering it with Kerdi, then combing the top half. Or run the sheets horizontally and cover smaller sections, whatever it takes to get everything done quicker. Using Hardibacker in this application is like adding calcium to wet concrete on a hot July day.
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Old 10-14-2017, 10:28 PM   #7
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What was the temperature you were working in? Was the water nice and cold?

I like to mix smaller batches to keep the thinset wet and creamy. Once you start noticing the thinset skimming and thickening it will kick very fast after the Hardie sucks the moisture out. Like others said a pump sprayer is useful here to mist the Hardie and keep it wet.
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Old 10-14-2017, 11:31 PM   #8
makethatkerdistick
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Kevin and John,
Thanks for your feedback. You confirm what I suspected.
I ran 8 foot sheets up to the ceiling, and perhaps that took too long although I will say I was within the times you specified.

It's currently 75 deg. F in the bathroom, warmer when I am working in it (as it is small). Cold water is hard to obtain as it was in the high 80s today (I am in Texas). If I keep running the AC to bring down the temperature, I'll also increase moisture removal. That won't help.

Next week we'll have colder weather. That might help. I guess my little handheld squirter won't do. I need to heavily target the hardie with more water. You're basically saying to do it until it is fully saturated, I take it.

Two questions:

1.) Will the thin layer (approx. 1/16th in) of thinset that I now have on my Hardie boards retard the water evaporation slightly (and thus help me)?

2.) As mentioned above, is a concrete sealer an option (to reduce water absorption to begin with)? Just something that will make the Hardie more manageable.

At this point, I want a good installation. I don't care about buying additional supplies if necessary. At least, I realize that I was cussing at the wrong product initially.
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Old 10-15-2017, 12:49 AM   #9
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Quote:
What do you guys think my mistake was?
Mistakenly thinking that you need to use Hardi instead of plain old drywall which is the tried and true method of building a Kerdi shower. Don't reinvent the wheel. Hardi doesn't make it "more" waterproof. Your Kerdi is either waterproof or it's not.
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Old 10-15-2017, 10:17 AM   #10
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Put some ice in your water before mixing.
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Old 10-15-2017, 02:00 PM   #11
makethatkerdistick
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Why didn't I think of ice! Brilliant!

I will give Schluter a call tomorrow to inquire about concrete sealers. I just want to make sure I won't have another failure. I was also thinking of putting on a thin layer of thinset onto the back of the kerdi, perhaps with a paint roller.
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Old 10-15-2017, 02:14 PM   #12
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Quote:
Originally Posted by Wolfgang
I will give Schluter a call tomorrow to inquire about concrete sealers. I was also thinking of putting on a thin layer of thinset onto the back of the kerdi, perhaps with a paint roller.
I wouldn't do either of those. The sealer will be a bond breaker, and the roller will probably be more frustrating than what you've already done.
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Old 10-15-2017, 02:40 PM   #13
jadnashua
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IF you get the Hardiebacker saturated so it doesn't continue to suck up water, you'll then have the full time of the mortar and things will flow.

Most sealers/primers won't work well on that super absorbent material. Many of them don't like fiber reinforced materials, either (HardieBacker has up to about 14% fiber in it - mostly cellulose as in wood fibers).
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Old 10-15-2017, 07:34 PM   #14
makethatkerdistick
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Thanks for all the feedback. It seems like maximum Hardie saturation is the way to go. I got my garden sprayer today.

I also saw the Primer T by Mapei at the store. Ryan mentioned this product in this thread. It is especially formulated to bond concrete surfaces it seems. Anyone ever tried it?

But water saturation might be my best bet for now. Just have to wait for my new Kerdi order to show up.
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Old 10-15-2017, 08:11 PM   #15
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Pull the Hardiebacker down and put up Kerdi board. Or Wedi.
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