If you are using a cement based adhesive then you will need to prime the plaster with a suitable primer i personally only use weber pr360 it is a acrylic primer and i use this type of adhesive on all walls now apart from on kitchen walls with small tiles then i will use tubbed adhesive.
Should you prime walls before tiling.
Previously painted textured drywall prep.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
Prime the area to seal the new drywall.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
When you spread tile adhesive onto a wall the water in the adhesive makes the pva live and stops the adhesive from penetrating the substrate and providing a mechanical grip.
If you re not sure if you have lead paint test kits are available.
If you can t remove the wallpaper your options are probably limited.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
You can apply a tile primer onto floors or even walls.
Or for painting again either primer or a thin skim coat.
Tile adhesives work by crystallising when it sets.
A tile primer should be used after the substrate has been deemed ready for tiling.
Basically your tiles grout and adhesive are being held in place by a thin layer of pva.
Or prime and paint the entire wall room before starting with your tile project.
How to prep walls before before tiling plus a sneak peek of the new kitchen tile backsplash.
Then once the dust and general debris of building has been cleaned up the best tile primer for your task can be used.