I've just taken off the old tiles in my bathroom. Unfortunately behind
the upper tiles is orginial plaster (horse hair etc.) This has crumbled
and most of it has come off the wall. It appears that lower down a
similar problem occured in the past, as this has been replastered.

There are no damp patches

As I am going to re-tile I want some advice on what to patch up this
wall with. I have considered using thistle hardwall. But wasn't sure
whether to then put a smoother top coat on as well.

Any advice and tips gratefully received.

Tim