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Old 10-15-2017, 08:28 PM   #16
rmckee84
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I used the primer T today on a budget job that already had cement board up and it worked fine. If I were you I'd just pull the cement board down and buy a couple sheets of drywall then cover that with kerdi.
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Old 10-16-2017, 12:07 AM   #17
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The way to go is pulling that down and putting up drywall.
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Old 10-24-2017, 12:34 AM   #18
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Just to give an update. Mapei Primer T saved my life. I put a thin coat on as per instructions (50/50 diluted with water). It got absorbed into the surface quickly and yet maintained the grab of the surface. The primer dried quickly, and after 2 hours I set to work with the Kerdi. Watering the Hardie could now be kept to a minimum.

What a difference. Everything went on smooth with great coverage! My Kerdi had completely turned grayish from bright orange, a sign that contact was being achieved uniformly. I occasionally pulled back to check and was very pleased. Open time of the thinset was outstanding and there were absolutely no ridges or tiger striping.

I have never been happier losing my manufacturer's warranty.

I like Schluter products, but if they include Hardie among their approved surfaces they need to develop a new, better protocol. Or at least issue a warning.

Thanks all for your insight and help. I just couldn't bring myself to tear those backer boards out. One has to choose one's battles.
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Old 10-24-2017, 06:22 AM   #19
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when you first installed the ditra, did you peel it back occasionally to check the adherence of the thinset between the substrate and the mebrane like the schluter video suggests? i plan to attach ditra to hardiebacker also, but now i am horrified by this thread. i used hardiebacker in the past, but instead of a membrane, i applied Redgard. i wonder if Redgard would be better than any kind of primer when using ditra? (i prefer hardiebacker to gyp drywall because it seems more structurally logical since the tiles are relatively heavy.)
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Old 10-24-2017, 06:30 AM   #20
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FWIW drywall is a perfectly fine backer for tile, so long as it's being used appropriately (ie with Kerdi or in a dry area). It might seem logical that cement board would be "better", but drywall is "good enough" so you're just making more work for yourself. I had a lousy 18 or so year old builder's grade drywall shower go bad on me, aside from the actual water damaged parts the rest was completely solid and a bear to tear down.

All these threads are convincing me more and more that Wedi/Kerdi board are the way to go for us noobs
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Old 10-24-2017, 06:59 AM   #21
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I had sticker shock when I saw the price of 1/2'' kerdiboard. i think it was 70 dollars for a sheet that was not even 96x48.
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Old 10-24-2017, 07:02 AM   #22
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wolfgang - the success you finally had with the primer - was it applied to a fresh piece of hardie, or to the old hardie with remnants of thinset disaster?
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Old 10-24-2017, 07:25 AM   #23
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No doubt Wedi and Kerdi board are expensive. I paid $50 per 3x5 for Wedi. IMHO worth the benefits though - inherently waterproof, about 1/4 the weight, truly easy to cut cleanly with a razor knife, essentially a single step installation. Wedi's a bit easier with the sealant at the joints vs. Kerdi board with the mortar and Kerdi bands, but both have got to be easier than dealing with the cement boards, rolling on a liquid membrane/applying Kerdi, etc especially for a novice.

FWIW I figured that for my 3x5 shower it cost about $400 for Wedi (6 panels, 6 tubes of sealant, 100 ct each screws and washers), vs. ~$220 for cement board and Redguard. My time may be "free" but it's also scarce, so the time savings and more fool-proofness of Wedi was the way to go.
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Old 10-24-2017, 09:01 AM   #24
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Frank, after I had torn down my first attempt at Kerdi, I skimcoated the entire surface with thinset to even out any previous trowel marks that had been created by the moisture-robbing action of the Hardie. It was a minimal coat just to get rid of the marks. So, technically, I bonded the primer T to that surface. However, it stopped the water thirst of the underlying Hardie. The funny thing is that I used a minimal amount of primer for the entire wall area to achieve that (maybe 300 ml primer and 300 ml water for about 70sqf).

As per its instructions, the primer T is approved for use on concrete products such as CBUs. It's meant for bonding to a self-leveling agent for floor application as I understand it. I don't see the drawback using it in my situation. I have never used Kerdi on drywall, but I figure the primer treatment gave me a similar experience.

You talk about Ditra. Did you mean Kerdi? Either way, they both have the same bonding fleece.

Some people here might advise against it, so pick your poison. However, I rather use the primer and have a great bond than struggle with the Hardie product and sacrifice integrity.

I did call Schluter, and I was impressed that they gave me the cell of one of their local reps. Certainly a good company. However, as nice as the rep was on the phone, he just reiterated the official Schluter protocol and would not sanction any alternative. In other words, if you can read instruction manuals, there is no need to have them read to you on the phone. So, here I am without a warranty but with a happy bond.

Since I had the primer applied, I decided to stick with my Kerabond thinset. Some people here have successfully applied Versabond. I didn't see the need for a slightly modified product.

ADDENDUM:
Frank, on re-reading your post, it seems that you have already applied the Redguard to your CBU? In this case, the primer might not be a good idea. I've never worked with Redguard so can't say anything about it. The primer is much more affordable, though.
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Old 10-24-2017, 09:00 PM   #25
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The Kerdi Instruction manual does call for wetting the cbu prior to applying the thinset and then the membrane. They do the same thing for any backer. The key is to ensure you actually do get it damp. WIth Hardiebacker, it takes a LOT of water to get it damp. You can tell from the surface. A single wipe with a sponge, and after moments, it does not look damp. You need to keep going until it does look damp with Hardiebacker. With drywall, the paper is quite thin, and the gypsum underneath isn't anywhere near as thirsty as Hardiebacker, so it doesn't take more than a single wipe, which also makes it easier.

The company tested their membrane with various backers, but they do (highly) recommend you use drywall. In the case where a cbu is already installed, or you insist, it DOES work. Something in the industry called SSD (I get this backwards sometimes, but saturated, surface dry). That helps with any tiling, including just spreading thinset to install tile.

Thinset, when mixed properly, assumes the moisture won't be sucked out which changes the consistency and the ease of setting/installing whatever over it. It must be fluid enough to spread, but not too wet to allow the aggregate to separate from the cement, which can lead to a weak bond. The thinsets do list a min/max amount of water to be used...one should NOT add more. It will work if you have SSD conditions when mixed properly within guidelines. It's an extra step wetting the surface, so some try to compensate with adding more water to the thinset. The results can end up less than ideal. It's lucky that modern thinset has an excess of capabilities, and things don't generally fail. Another reason why Schluter calls for the use of a premium dryset...using an entry level one may not have that margin for error.

The reason using a primer makes things easier is that the surface is already saturated...not because you added water, but because it has sealed the surface, and it does not absorb any moisture from the thinset...saturating the surface with simply water in this case, would have done the same thing - prevented the board from sucking moisture out of the thinset and preventing it from spreading and flowing properly.
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Old 10-24-2017, 10:05 PM   #26
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Yep, you got SSD right this time, Jim!
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Old 10-25-2017, 07:24 AM   #27
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Don't mean to hijack the thread, but I want to clarify for myself. The overall consensus is to use Kerdi board (or the like), drywall or cement board (with the Kerdi membrane)?

Additionally, am I ok to tile over drywall for the rest of the bathroom walls or should I hang hardi or concrete board?

Anthony

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Old 10-25-2017, 09:41 AM   #28
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jim - thanks for the detailed info.
wolfgang - so now i have a dilemma. do i first skimcoat dampened hardie and then apply primer to mimic your ill-intentioned method, or do i just prime the plain hardie before hanging the membrane with thinset? it's a rhetorical question and i don't expect a reply. and no, i have not applied redgard - it was only an idea. right now, i have unaltered .420'' thick hardie installed and am awaiting arrival of the shower kit.
furthermore, i was disturbed by the hollow sound another member mentioned about the foam pan. the old shower pan was plastic and you could hear the water impingement downstairs when someone takes a shower.
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Old 10-25-2017, 09:29 PM   #29
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Adding a layer of thinset to the HardieBacker is, IMHO, a waste of time and materials! What you DO want to do is the get the backer quite damp. Keep wiping it with a wet sponge until it doesn't change color or accept any more. As I said earlier, some people just spray it liberally with a garden sprayer full of water. As long as there's no liquid water pooling, it is not too wet, and if you do get it that wet, just wait a few minutes. If you do this, the board will not absorb moisture from your thinset and make it stiff and thus hard to embed the membrane into the thinset. When the thinset is the proper consistency, it is MUCH easier to install the Kerdi properly. That won't happen if the substrate sucks all of the moisture out of it. The difference is almost like day and night. FWIW, it's easier to get drywall damp enough, and is one reason why it is the preferred backer.
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Old 10-26-2017, 09:13 AM   #30
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I agree with Jim that the layer of thinset will not help in any way. It won't change the property of the Hardie below.

Theoretically, the garden sprayer should work. I've done one section when I redid my project where I liberally kept watering the Hardie with just that, a garden sprayer. And yet I ended up (albeit less this time) with a few unattached areas that I caught early and recoated. That's when I switched to the primer.

Maybe if one does this on a regular basis, the routine will help. In my case I opted for the primer which was quicker and didn't entail any guess work as to when the Hardie was saturated and how long an open time that would create for me. Plus, the garden sprayer does create more of a mess than the primer.
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