If so you will need to paint the wall with pva glue tip some ova into a bucket and thin it down a little with water so you have a creamyish texture then paint onto the area you wish to tile and allow to dry it should be dry enough to start tiling after half hour so start tiling fro the bottom and work up true ova seals the plaster and will give you a surface for the adhesive to adhere to hope this helps.
Tiling over plaster walls bathroom.
It used to be very common to put tile up on whatever the wall was and seal the the tile.
You may not have lathe but compressed board under the plaster and scratch coat.
That is true you cannot tile over plaster if that s your plan.
The adhesive and tile will block moisture from evaporating.
Bring plumbing to code.
You would have to remove the plaster and replace it not with drywall run from a contractor who says that.
Follow manufacturer instructions to a tee.
This was back in the 80 s.
But with cement board.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
Fresh plaster whether applied as a patch or to the entire wall must dry completely before tile is applied.
If you have a tiling job to do in your bathroom and the walls have been skimmed with plaster you have.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
If plaster is any way compromised you should tear out to studs and start over.
Tiling over damp plaster causes the wall to fail because the plaster won t become strong if it can t dry out.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
I have repaired tile jobs that were over 20 years old that the tile was applied directly to regular sheetrock and plaster walls.
Add a diverter and shower line in the wall.
It is important to check the weight of your tiles as plaster skim has a 20kg per square meter weight ratio.
The problem otherwise is that grout is porous and any slight crack will cause moisture and mold on any other surface.