Would like your opinions on this one please. Got a bath/shower installation to do soon, and the existing walls are painted plasterboard (not skimmed). The approach i would normally take is to remove the walls replace with Hardie and tank the wet area.

The problem I have this time is that the plasterboard is that eggbox stuff (Paramount is it?), and i don't really want to remove it. The paint appears well-bonded to the plasterboard - nothing comes off with the gaffer tape test. So I would like your opinions on the following approach please. Sand down and key the painted plasterboard, prime with BAL APD, tank with WP1, and tile. What do you think?

Of course I've seen threads re tiling onto painted plasterboard before, but I haven't seen anything relating to tanking onto it.

Looking forward to your replies