loose plaster, best prep for small tiles

[jape]

Ripped off the old tiles round bath, one wall is painted plasterboard so i will scratch, prime and tank unless convinced otherwise; the short wall is old grey plaster, solid in places, loose in others with a badly done pink skim over most. I wish to avoid plastering this whole area so intend to knock of loose and patch, then prime and tank.

I have a bag of Mapeker rapidset. Would this do for patching? I have seen tiler using it quite thick filling holes as he went then coming back as he tiled straight over it in similar circumstance. He said he'd been doing it for years, no problems. I assume compatible with primer, tanking (also Mapei kit) if I use it for a base under them. I will use a slower set flexible adhesive as I am not fast tiling.

So, if not a good idea whats ok for patching? Mortar maybe? Polyfilla?
And recommendations for adhesive on tanking please.
I have used Diall all in one successfully but most here say not to and as shower over bath I want good adhesion and a waterproof grout too, well as waterproof as they get.
Thanks. Tiles are cheapo 150mm plain white.

[steviejoiner74]

If the plasterboard is painted I'd consider replacing it,it's only a fiver a sheet. This will give a sound surface to tile to.
For any areas with painted plaster which needs the odd patch I'd consider painting blue grit over the surface.