They were built by the joiner using 18mm ply so that the mosaics fitted perfectly, only thing I had to worry about was spacing them from floor to ceiling, easy to lose or gain a bit over 2.8m.
Edit; sorry misunderstood. They were ok on the sheets, still had to lay them out dry on the floor first. Some had fallen off the sheets and had to be put in individually! The hardest bit was lining one sheet up with the next.
When I did my bathroom with mosaics I used the alloy trim from CTD. It was a struggle but I simply used a thicker bed of mosaic fix.
I have to say I like the idea of a skim of rapid set to bring the moseics out level though.
A good tip that was passed to me and worked was to interlock the mosaic sheets by cutting out alternate mosaics from mating sheets so that they mated together. If the odd piece falls out, stick it back in when the rest of the sheet is on the wall. That loses the obvious join from one sheet to the next.