QUESTIONHi, I am getting ready to tile the walls around my new bathtub. We already replaced the walls with hardiebacker and after I applied a wateroof membrane, Aquadefense. I had a guy come over to help me with the tiling, and he noticed that the walls were warped a little. His solution was to apply a mixture of thinset and specmix to even out the walls. He applied it yesterday, and is planning on coming over today to tile over it. Is this ok to do? By the way, we are using porcelain tiles, 12 X 14 pieces.
ANSWERANSWER - What should have been done while installing the Hardibacker board on the walls, is the framing studs should have been shimmed and adjusted to make sure the Hardibacker board was plumb and square.
If the walls were not too far out of wack, you can use the thin-set mortar or there are special patching mortars for going over the Hardibacker board as long as it isn't applied too thick. That should be done before you applied the waterproof membrane.
I'm not sure what the specmix was other maybe it was a pre-mixed sand/cement mortar, which could have worked if it was done correctly. It is possible to adjust the backer board surface over the membrane, but it can be problematic if not done correctly. Plus you won't have crack isolation protection over the repaired portion of the backer board.