If plaster is any way compromised you should tear out to studs and start over.
Tile over plaster walls in shower.
Add a diverter and shower line in the wall.
Install the diamond mesh directly over your tile surface.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
The mesh composition of this material will serve as teeth and lock in plaster applied to its surface.
It used to be very common to put tile up on whatever the wall was and seal the the tile.
Tile on tile or as i say it.
Use 7 5 cm 3 inch galvanised screws and screw directly through the mesh and into the wall framing behind the tile.
Tiled walls can last many years but that s not always a good thing.
Cover with cement board tape off.
Or flatten walls with a latex modified skim coat product.
Bring plumbing to code.
The surface of a plaster wall is too smooth for the cement to grip to.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
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Colorful tiles may go out of style and some tiles are inappropriate for the space.
Follow manufacturer instructions to a tee.
Some products are designed specifically for use between tile and plaster walls but any tile setting adhesive designed for thin set use will work.
If you re not sure if you have lead paint test kits are available.
This was back in the 80 s.
A glue adhesive works better for installing tile over plaster.
How to tile over existing wall tile.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
This is a renovation i did of my 1960 s plaster mud job bathroom.