[jeffnc]

OK, so let's clear up some issues here.

First of all, you're using Kerdi board. Gcraay used Kerdi membrane over drywall (apparently, his explanation seems odd.)

You would not want to use a "Kerdi brand trowel". Assuming he means a Schluter brand Kerdi trowel. But that trowel is 11/64" or 4.5 mm, not 1/8", so not clear what he's talking about really. A Kerdi trowel is meant to install Kerdi membrane, not tile.

Just so you know, smaller tiles will not be as strong on Kerdi Board as it will be on drywall or cement board. However a 3x12 tile will be fine. The strength of a Kerdi install comes from the complete assembly, not just the individual materials. So the thinset actually strengthens the entire wall. You would want to use a 1/4" x 1/4" trowel size. That's assuming your wall is fairly flat. If you have waviness you can consider a larger trowel size (but better to make the wall flat first). I would recommend a good quality thinset such as Schluter All Set. If using other brands, it's more complicated.

It's unlikely he was actually back buttering his tiles, because that really isn't necessary in this case. He was probably back troweling. Back buttering is putting a thin layer of thinset on the tile before installing onto the surface that already has the troweled thinset on it. You can either trowel the thinset on the wall, or onto your tiles, but not both obviously. If you put it on the wall, just make sure you install tiles before it has a chance to sit on the wall too long and skin over.

 

[jeffnc]

I’m back buttering a thin layer to get a better hold. Especially in the shower. I’ve heard you want a 95% contact. And because I have so many, I figured I really don’t want to have any issues. Plus the back is a waffle pattern.

I think you're a little confused about the meaning and purpose of that. You don't want 95% contact, you want 95% coverage (actually 100% is what you want). But the reason is completely different in a shower wall than a floor that you'll be walking on. On a floor, you want full support. On a wall, you don't care about support, you just don't want any voids where water can pool up.

In any case, back buttering is about adhesion, not coverage. If you don't use enough thinset, of the right consistency, and fully compress the tile while allowing air to escape, you'll never get full coverage regardless of how much back buttering you do. On the other hand, if you use enough thinset, of the right consistency, and fully compress the tile while allowing air to escape, you don't need back buttering. Tiles aren't falling off shower walls, so ultimate adhesion really isn't an issue, and in any case, back troweling solves the problem just as well anyway, and is much faster overall

 

[jeffnc]

Sorry, my “Schluter” trowel. It actually says 1/8 x1/8 on it. And yes I’m back buttering every single tile.

My mistake on the dimensions - I was looking at a Ditra trowel when I wrote that. So yes a Kerdi trowel is 1/8" x 1/8".

But again, that is definitely a bad trowel size to be using with 4 x 12 subway tile. You want 1/4" x 1/4". A Kerdi trowel is not for installing tile over Kerdi, it's for installing Kerdi itself.

And regarding the back buttering, see my previous comment.