I have just finished my kerdi installation and have some advice....

1 - don't use hardibacker - use either the recommended sheetrock, or Mongo uses Durock quite a bit. Hardibacker made it infinitely more painful for me b/c it makes the thinset skim over too quickly.

2 - Use non-modified thinset so you don't get the lecture from the schluter reps if you call them with questions. I used versabond (slightly modified) b/c everyone at johnbridge tile forums uses versabond. Seems to be fine.

3 - Don't squeeze too much thinset out from under the kerdi. I think I probably squeezed too much out especially around the drain area. Oh well. I'm getting some kerdi fix to seal things up better and be sure.

As far as how smooth it is? Not sure what you mean. If you make your thinset a good bit runny, you can mold the kerdi however you like. I've heard of some using rolling pins to embed it in the thinset. I used a big concrete float to press it in. Mongo uses a drywall knife. I have a few negligable ridges/humps here and there but nothing that can't be evened out with thinset on the tile surface.

i did my first tiling last year & used kerdi with the help of bill & mongo. it sounds like your husband is concerned about getting the kerdi flat enough to be able to tile over it? the key to that is making the thinset under the kerdi quite wet/loose, working in manageable-sized areas depending on his comfort level & speed, and using the durock like nc said above & pre-wetting it before applying thinset under the kerdi. have him check out mongo's giant post & ask for details here if he's still worried. there is actually a very nice picture that shows the consistency you want the thinset under the kerdi, and a lot of detail about how to smooth it, etc.

Here is a link that might be useful: mongo's instructions

My Kerdi is up... and staying that way!

My first sheet was definitely not as good as subsequent ones, so I wish I had had a test wall I could have tried one first.

I also made my thinset a bit too runny, which prevented the problem of skinning over, but was soooo messy... so thin, but not too thin:) By the time I got down to the bottom of the wall, I had to sit on the floor... which had thinset all over... like I said, I made a mess, but luckily I just rolled up the newspaper and threw it out.

The correct trowel puts just the correct amount of thinset onto the wall, so that when you smooth it out, there shouldn't really even be enough to make too bumpy at all!

I used a 12" drywall knife that I thought worked very well and the size allowed me to evenly and quickly bed the Kerdi into the mortar.

So, with all my concerns about trying to install the kerdi, that was not at all one of them, though I too had many.

I studied Mongo's tutorial, which was key, and I also read John Bridge's book. But really looking closely at Mongo's photos and reading his descriptions I think is the only reason I even thought I could try.

Schluter states you should use unmodified, which really makes sense if you understand how all the parts works, at least to me.

Tell him to use a drill and paddle to mix the thinset. No lumps that way. ;-)