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jeffnc 01-11-2013 06:20 PM

How to "key in"
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"Keying in" is a basic concept that applies to many applications. In general, you are applying a liquid or semi-liquid/wet substance to some solid surface. The point is to achieve great adherence and more consistent, longer lasting results.

It's difficult to show because often keying in occurs at the microscopic or near microscopic level. Three common examples are paint, joint compound, and thinset. Using paint over drywall as an example, what are we trying to adhere to exactly? We're trying to adhere to paper. If you look at paper under a microscope, it looks like this.

You can see all the nooks and crannies. You can imagine how getting the paint down in there would greatly increase adherence and scrubbability. So when applying paint, we're really trying to accomplish 3 things:

1. get the paint from the container to the surface
2. key in the paint to the surface
3. create the desired finish

Sometimes you can accomplish 2 or even 3 of these at once. For example when rolling paint on a wall, filling the roller cover and then rolling the paint on accomplishes all 3, assuming you use a modicum of pressure to push the paint into the surface.

When applying paint with a brush, such as polyurethane to a wood table top, the paint brush and gravity combine to get the paint into the wood (1 & 2). (And polyurethane is thinner than those thick acrylic wall paints.) Then you pull the brush over smoothly in long strokes (the length of the table) to get the most consistent finish (3).

When painting a house with siding, some good painters will apply the paint with a sprayer (the most efficient way to accomplish 1) and then brush it in (the best way to accomplish 2). Sprayers are great at moving paint from the container. They suck at keying in the paint.

When applying an epoxy to a garage floor, the paint can be poured onto the concrete (1). Then a squeegee is used to firmly spread the paint, keying it in to the microscopic craters in the concrete (2). Finally a notched squeegee applies the paint with the correct thickness (3) allowing it to self-level properly (3).

jeffnc 01-11-2013 06:25 PM

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This sequence shows installing Ditra. I like this example because the holes in Ditra are very large and can easily be seen with the naked eye, so it illustrates the idea well.

First, when applying thinset to a plywood subfloor for the Ditra, the thinset should be moved from the container to the surface either by pouring it or by "shoveling" with a trowel (1). Next, using the flat side of the trowel, spread the thinset well over the surface firmly, "rubbing" it into the plywood. Notice the steep angle of the trowel. This keys it in, filling in the nooks and crannies (2).

(This same approach is used when applying thinset for Kerdi on drywall - it gets keyed in to the paper.)

jeffnc 01-11-2013 06:27 PM

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After that, build up a thick layer of thinset by spreading it around with the flat side of the trowel (begin 3). Notice the flatter angle of the trowel.

jeffnc 01-11-2013 06:27 PM

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And spread normally (3).

jeffnc 01-11-2013 06:30 PM

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When putting thinset on top of Ditra to begin tiling, we have the same situation. Pour some thinset onto the Ditra (1). Spread it fully with the flat side of the trowel to fill the waffle holes completely (2). If your waffle holes look like the following, you have not properly "keyed in" the thinset to the Ditra.

jeffnc 01-11-2013 06:33 PM

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Here the thinset has been well keyed in, and a thicker layer is spread before finishing (begin 3).

jeffnc 01-11-2013 06:34 PM

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Now you're putting on the "finish" (3). In this picture, the thinset hasn't been applied thickly enough, and the ridges aren't fulling formed.

jeffnc 01-11-2013 06:37 PM

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Better ridges and a finish coat (3) on top of a well keyed in material.

Keep the Ditra imagery in mind the next time you're painting or doing some other "invisible" work, to get better results with a material that should be keyed in.

kt82 01-13-2013 09:05 AM

what if in 5 years you( read spouse) want to change tile? .Will the "keyed in Ditra " be removable with out removing the subfloor
I watched the Schlutter video on Ditra and it looked like they applied the thinset right into the OSB.

jeffnc 01-13-2013 10:04 AM

It will be removable. Of course it's not going to come up really easily, but that's pretty much the point. It's supposed to be kind of hard to remove, just like it should be hard to knock down the walls of your house or get paint off your walls. You can rent a tile removing machine, but for a small area like a bathroom it wouldn't be worth it.

Good question though. If an area gets very light traffic and you think you'll want to change tile in a few years, as a homeowner you might even want to use shoddy installation techniques to make the tile easy to get up! It's not something that anyone would recommend, but for the pragmatic homeowner with a tendency to change designs often, it can work. For example, when you press your tile down into the thinset, the thinset is supposed to spread on the tile to cover at least 80% of the bottom of it (it's a good habit to pick up a tile now and then as you're laying them down to look at how the thinset is covering the bottom of them.) If you want them to come up easy, don't press down hard and just go for 50%. Of course don't complain if a problem does arise, but with very light traffic - it really probably won't. Half the tile in bathrooms is never walked on anyway.

If tile is hard to get up, I use a demo hammer like this one

(To rent one costs almost as much. But you can use a hammer and chisel as well.)

But the Ditra itself isn't going to be as hard as tile to get up, so it doesn't make sense to not adhere that properly to the subfloor wood. For a tough time getting up something like that, I use

Over something like OSB, it's possible that you could bring up little (coin sized) chunks of wood in places. That's not a problem, depending on what you're putting in next. If it's tile again, then the thinset will fill in those little areas.

kt82 01-13-2013 10:15 AM

So i can remove the tile and the properly embedded Ditra will stay adhered to the OSB.
and I can install new tile over the old Ditra.
My concern was the Schlutter video showed the Ditra installed directly onto the OSB subfloor .
Am I correct? Thanks
( I have the bigger HF demo hammer and have used it for 4 years )

jeffnc 01-13-2013 10:23 AM


Originally Posted by kt82 (Post 1092335)
So i can remove the tile and the properly embedded Ditra will stay adhered to the OSB. and I can install new tile over the old Ditra.

I'm sorry, I didn't understand your question at first.

Actually I have no experience with that. My guess is that it would be problematic, because it just seems to me that taking up the tile would tend to pull away the Ditra from the floor in some places. But I don't know. If you're careful taking up the tile and the Ditra looks intact and you're not doing anything that needs to be guaranteed, then I think you can go for it. I doubt Schluter recommends or warrants it though. At the very least there's a strong chance the Ditra would at least be nicked or cut in some places, making it not waterproof.


Originally Posted by kt82 (Post 1092335)
My concern was the Schlutter video showed the Ditra installed directly onto the OSB subfloor. Am I correct?

I'm not sure which specific video you mean (have a link?) but Ditra can be applied over OSB. I'm just not sure what your concern is exactly about the video.

kt82 01-13-2013 11:14 AM

thanks for your help

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