Plaster prep around small shower area?

[mortarbo]

Due to an illness, a new shower was recently fitted over the old bath.
The walls were 30+ years old plaster (still looks ok) covered in waterproof paint.
The volume of the shower etc has removed the paint down to plaster (now gets wet but dries quickly over couple of days) this last few months.
I want to tile in this key area side of bath. What treatment do I need to add to the uncovered plaster before putting in some tiling about 1 metre wide full height?.
It's not a big area as the info above indicates. Thankyou

 

[Wayners]

Scratch up the area back to plaster. Use a tile primer to seal surface

 

[ree]

OP,
Why not post pics of the walls?
Are you talking about masonry walls, stud walls or a mix?
Has plasterboard been used?