Not all plaster wall surfaces will support the tile properly.
Tile over plaster walls.
Don t use paint glue pva wall paper paste only use the primer recommended by the adhesive manufacturer of your chosen tile adhesive.
Also now it s been plastered you will be restricting yourself to a tile with a max weight of about 16 kg m2.
You would have to remove the plaster and replace it not with drywall run from a contractor who says that.
Adhesive grout and tile 20kg max weight onto plaster.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
It used to be very common to put tile up on whatever the wall was and seal the the tile.
Layer newspaper on the floor across the bottom of the tiled wall.
Add a diverter and shower line in the wall.
If you re not sure if you have lead paint test kits are available.
Mold does not grow on cement.
Then rinse the wall with a water dampened.
If plaster is any way compromised you should tear out to studs and start over.
Follow manufacturer instructions to a tee.
Scrub the wall tiles with diluted degreasing cleaner and a nylon scrub brush.
Loose paint must be scraped.
I have repaired tile jobs that were over 20 years old that the tile was applied directly to regular sheetrock and plaster walls.
Cover with cement board tape off.
Mix degreasing cleaner with water in a bucket using a dilution recommended by the cleaner manufacturer.
The problem otherwise is that grout is porous and any slight crack will cause moisture and mold on any other surface.
But with cement board.
This was back in the 80 s.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
Testing the wall for durability.
Both painted and bare plaster allows for the installation of tile.
That is true you cannot tile over plaster if that s your plan.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.