ᐅ Plastering the bathroom / leveling uneven surfaces

Hello,
I’m new here and have been reading for a while. Now I have a question that I haven’t found an answer to in this forum yet.
I am renovating a house (built in 1963).
I have gutted the bathroom.
The old tiles went up about 1.20m (4 feet) high and were installed using a thick-bed method. The wall above the tiles was wallpapered.
Later, I want to tile up to the ceiling, but the thick-bed adhesive protrudes about 1cm (0.4 inches) beyond the plaster above. So I need to level out this 1cm (0.4 inches)… but what would be the best way to do this?
I think the old plaster is a lime-cement plaster, with a yellowish-brown appearance.
Will a new plaster layer about 1cm (0.4 inches) thick adhere well there? Or what other options do I have?
Thanks in advance.
Regards, Michael

ich74 schrieb:
..... or what options do I have?

Remove all the old stuff completely! A one-time effort. Anything else is just patchwork.

Best regards

Exactly, get rid of it – in my opinion, there’s not much worth saving, and the effort wouldn’t be justified in the end. You would likely end up with a questionable result at the end of the day.

Omega schrieb:
Exactly, get rid of it

Hello, first of all, thanks for the answers.
Remove all that thick-layer stuff...? It’s about 4 to 5 cm (1.5 to 2 inches) thick and much harder than the wall behind it (lightweight concrete).
At first, I thought it would be faster to remove the tiles along with the thick layer than just the tiles... but not really. Parts of the wall broke off because it was so hard/firm.
Regards, Michael