Backerboard/tanking/dot and dab wall

[AverageDIYerEssex]

Hi all. So I'm about to start re-tilling my bathroom. I have a section of wall where the 1700x800mm shower is going that is MR board that I want to either board over or replace with some form of backer board. Be it cement or the foam board ones.

But one wall of the shower (the 800mm side) is dot and dabbed plasterboard, plastered and painted. How do I secure the backer boards to this? Or is there another way to achieve this some form of water protection?

Also would chipping the plaster off of the PB and then tanking it with the Mapei shower kit instead be an option?

Many thanks.

 

[Deluks]

I'd be inclined to lose the plasterboard, and replace with cement board before tanking and tiling.

 

[ree]

OP,
As above, remove all plasterboard in the wet wall areas and screw the backer board into the masonry or stud walls. The tile comes about 100mm past the edge of the shower tray.