Hi Bill,
How are you building your shower enclosure? For the most part, a medium-bed mortar isn't necessary when setting wall tile, but a non-sag formula is helpful. For that reason, the 4XLT product *might* be appropriate, depending on the surface it's being installed over. 4XLT takes a long while to cure and being sandwiched between 20" porcelain and an layer of waterproofing might just keep it pastey (technical term) for a long while. The "dot method" is useful but you still need to make sure that you've got a coat of thinset burned into your substrate, or your bond is only superficial. Laticrete's "SureSet" seems to be their Lowe's-only medium-bed and I've heard from other installers that it's a good product. I haven't tried it yet.
Best of luck,
Shaughnn
Every thinset manufacturer offers a medium-bed mortar, but they almost all recommend using a larger-notched trowel for larger tiles. A 1/2"-loop trowel, with a U-shaped notch, is usually a good bet for larger porcelain like yours, and you want to burn a coat of thinset into the backs of the tiles.

Shaughnn, what non-sag thinset would you suggest? I finished tiling my shower with 12x12s (my first tile work ever) but have one more to go. I had some frustration with the tiles sliding slightly and/or sagging out at the base from time to time. Thanks.

For 12x12's Laticrete's 255. If the tiles slide, you're not mixing it right. I've hung granite tile with it before.

Hello Prickly,
I can't think of a mortar manufacturer who doesn't offer a non-sag mortar formula now. I like Laticrete products but if they aren't readily available to you, then that recommendation doesn't do much good. Were I you, I'd contact my local suppliers and see which brands they offer, and which non-sag products are available to you.
If your bottom-course was sliding out it's most likely because either the thinset was mixed too loose or the thinset was applied too thick. Sometimes, little tabs of thin rough cardboard can be put under each tile as a kind of "traction stop" if you are installing tile over a dramatic tub cove.
Shaughnn

2 questions:
1. Shaughnn mentioned a sandwich, which I assume means a place with insufficient air, which inhibits the thinset from curing, keeping it pasty. But as a cementitious product, does thinset need air to cure?

2. I didn't know you could ever set large/heavy tile on a wall without some sort of support for the bottom row and tile spacers to support upper rows. So the thinset holds so well these support measures aren't ALWAYS needed?

Thanks;

Bill

I did not have a great deal of sliding, but some slight sliding on edge pieces once they were set (mostly sliding into the corner grout joint). I also had some trouble setting tiles in an area where the framing was off by a bit and I was doing thicker thinset to compensate. Maybe I should have skimmed a coat of thinset first, then come back and tiled over the dried coat? Sorry, I didn't mean to hijack the thread, just wondered if I had been using the appropriate thinset.

No problem, cactus, we're on the same topic. So you didn't use tile spacers - the tile just hung by the thiset alone, I mean other than the slight sliding you describe?

Yup, straight walls are necessary, especially for large tiles - I read that 1/8" in 10 feet is the tolerance. I'm going to straighten any studs, (plane/sand down high spots and add material to the low spots), to make a flat plane and then shear it with 3/8" structural plywood before putting up the CBU. I hope this will both make for a flat and strong backing for the tile.

I did use spacers. I had two different types actually. My tile store recommended some that you leave in permanently between the tiles, and I also picked up the standard ones you remove. And once I realized that some of the tiles were sliding, I used masking tape to tape end pieces next to the adjacent piece to help keep the grout joints tight. The tiles certainly weren't sliding off of the wall, but they sometimes would sag a little one direction or another. I used a ledger board to start on the 2nd row on the wall (the first full tile row).

The walls were mostly straight. In fact, we removed and replaced a few of the studs and planed others. The biggest problem was that we ended up removing and reinstalling the tub deck and not getting it quite plumb. As a result, the bottom was slightly farther back than the top. There were also just a few studs that we couldn't quite get perfect.

At first I also had some difficulty getting the appropriate consistency of thinset. My first batch was too dry and very hard to work with. A few later batches were probably a little too thin. I improved slowly (as did the result). I expected things to be a little imperfect (especially compared to a true professional), but I enjoy renovating things myself and learning new skills.

I will also say that as a first time tiler, tiling the horizontal surfaces was much easier and faster than tiling the vertical surfaces (and surprisingly less physically straining). Following instructions in several of the tiling books, I did the vertical surfaces first (so as not to stand on new ungrouted tile), but maybe I should have done a powder room or something first.

In the end I am still pleased with the result. I am certain experts like Bill V or Shaughnn could easily point out some errors (in fact I can too). But overall I think the project turned out well.