Tiling new plaster walls

[martyw]

Hi

I have recently had to have the walls in my on suite re plastered due to the previoust tiles (put up by previous owner) leaking and made the plasterboard mush.

My questions are:-

Do i need to seal the new plaster to put on new tiles? if so whats the best way?

Also the new plaster has a small bow down the middle so when you put a straight edge on horizontaly the middle sticks out, its fine verticaly. So i am worried when i start the tiling that the tiles will not be uniformly level accross the wall, what should i do to make them level?

Thanks in advance
Marty