Seal the walls with a skim coat and let it dry thoroughly.
How to wall tile on plywood.
You cannot install tile directly onto plywood because it is not dimensionally stable.
Mark the tile where it overlaps the installed.
Spread thinset adhesive with a 1 4 inch notched trowel over the plywood subfloor.
Clean and heavily score area to where tiles are to be fixed it sounds like the adhesive does not have a rough surface in which to secure itself to.
If you are tiling on a plywood floor with tiles above 400mm x 400mm in size or heavier tiles made of stone we recommend using an ultra flexible adhesive s2 such as bal fast flex.
If you put tile directly on top of plywood it will chip and break.
If possible you should always use concrete board underlayments rather than plywood.
You want the tile adhesive to have something to grab onto.
Use intervening layers of two sheets of plywood.
To be sure maybe drill up to 6 evenly spread out holes in the plywood at least 4mm deep or straight through in the areas to be tiled so the adhesive sinks into the holes and hardens hence creating a stronger fix.
Measure perimeter tiles by laying the tile flat on the surface so that it touches the edge and overlaps the last full size tile installed on a row.
Then you want to install screws into the concrete board in each of the holes.
For wall installations plywood is overkill beneath the concrete or fiber board which is designed in five eighths inch material for wall settings.
Sand with a medium grit sandpaper to create a uniform surface that s not too smooth.
The preferred method for tile installations is to use a sheet of underlayment.
While tile can be laid on plywood do not install tile directly onto the plywood subfloor itself.
Use thinset with a higher latex content thinset is the wet base that you first trowel on the plywood to make the tile stick.
Set the pieces of backer board into the adhesive.
To install the concrete board apply tile adhesive to the back then press it down against the plywood.