I had to have my old three-handle tub faucet replaced by a plumber, because the valves in the handles were rotted into the handle bodies. When the tile came off, we found that the wall behind the tile was not backerboard, but the old-style cement laid on a wire mesh. So, the plumber had to punch a hole in the cement to get to the pipes. Coming in from behind the wall was, unfortunately, impractical because there's a double sink in another bathroom on the wall behind the shower, and moving that out of the way would have been even more trouble. So now my new faucet is located in about a 16" by 14" (four tiles wide by about three and one half tiles high) hole surrounded by cement, which I have to suspend tile over.
The plumber did a pretty good job of making a square hole in the cement, although it's not possible to make a perfect square when chiseling cement, and he installed backerboard into the hole for me. So now I've got backerboard cut and installed, but there are smallish (about 1/4") gaps where the cement isn't perfectly square with the backerboard. The backerboard comes out to almost the right depth, although it looks like the new tiling is going to be sticking out a little bit compared to the original tile.
I'm pretty comfortable trying to install the tile myself, I'm mostly just worried about the gaps between the backerboard and the cement, and how to deal with tiles that will stick out a little bit from the rest of the wall.
The faucet, and the hole, is located directly on top of the tub.
So, my questions:
Will thin-set mortar suitably fill in the gaps between the cement and backerboard?
Is there anything special I need to do to prevent leakage on the transition between the new tiles and old tiles, where the new tiles are going to stick out about 1/8" from the old tiles, or can I just apply grout liberally?
I've placed a vapor barrier (3 mil plastic sheeting) between the first and second layers of backerboard. I tucked the plastic sheeting around the rear piece and poked the sheeting into the gaps between the backerboard and cement around the sides and top of the backerboard. At the bottom where the hole meets the tub body, I've let the plastic sheeting lay over the curved "drip edge" of the tub, so any condensation that might form between the cement board and sheeting should drip back into the tub. Should I do something different with the sheeting before I put mortar into the gaps?
I'm really hoping that I don't have to take down the whole side of the wall where the faucet is installed just to make everything uniform. Honestly, I'm okay with a solution that would drop tiles in 5 years time (but not a solution that would leak into the wall, obviously), I just don't have the budget, time or experience right now to undertake a "big" project. Alternative solutions that fit in my constraints are also welcome, e.g. perhaps I could get some more wire mesh to put cement on to more closely match the original wall?
Absent any more answers, I'm going to proceed with plan "A" - use plain thin-set mortar to fill the gaps and let the new tiles be proud and just use extra grout on the raised edge.