The structure of these interior walls is interesting; they're plaster on sheetrock, which is how plaster walls are done today. I don't know when it was phased in, but given my razor blade archaeology, I'm saying immediately post-WWII. Anyway, what we have is about 1/2" of plaster on about 3/8" of sheetrock.
The patch done earlier hasn't survived even a bit. It was 3/4" drywall on a thin piece of wooden paneling added as a shim to get to the total thickness of the original wall, but the drywall didn't survive the removal of the tub surround. Plaster survives everything except leaching by water. The parts that weren't soft were, well, rock-hard. Excellent craftsmanship.
Now, of course, instead of drywall, we use a cementitious backer board. Lowe's had convenient half-sheets (fit in the van) in 1/4" and 1/2" thicknesses. I thought I'd have to shim out another 1/8", but as we'll see, that 3/4" total thickness seems just about right already.
Our first step is to square off the hole. This was somewhat tedious, but fun, as it involved hammering bits of wall out with a screwdriver. (It was tedious because it had to be done accurately.) It took maybe 45 minutes, and the newly rectlinear hole already looks more presentable, doesn't it?
Next, we cut a piece of 1/4" backer board, painstakingly make the right holes for the faucet, and screw it in place with just a couple of screws. After all, we're going to be putting more screws in for the second layer.
Note that I've left a little open strip there. Since this backer board doesn't actually have to support anything (like plaster or tiles) we can get away with that; it's just there for thickness. Same with the hole a foot higher between the two strips of sheetrock. If we were plastering this wall, we'd have to do something about that, but in this case, we don't.
Cutting those holes in the right place was very difficult until I realized that just using a knife was not the way to go. After I used the drill to drill lots of little holes around the rim of each larger hole, it went pretty well.
Now here's the weird thing. I set it in place just to eyeball it before putting in my shims - and it's already thick enough. So much for measurement - go figure.
The next step is to do this again, with the 1/2" board over the larger portion. I'll update this post if I do it tonight; otherwise it will be a new post on the morrow.