ben wrote:

[wants to skim semi-external loo]

The paint won't have to go, if its as he says (the brick is showing through
the brush strokes), then the plaster will penetrate through this and aquire
a firm bond to the brickwork as it drying out.

Can you explain the mechanism behind your theory? Do you understand
exactly how plaster adheres to its substrate? Are you, or have you ever
been, an adhesives chemist? A basic test for paint adhesion is to
crosshatch it with a sharp blade at 2mm spacing over 20mm square, firmly
press down sellotape and then after at least a minute (elasto-plasticity
comes into play)pull it off sharply. If a single flake of paint comes
off it should not be trusted as an adherend.

Bonding plaster is the correct technique first and the a skim of finishing
plaster, although if he does a smooth job with the bonding plaster there
will be no need for finishing.
I recommend he use dry wall plaster for the bonding.

I was thinking of fixing plasterboard, but the toilet in question may well
have been designed on the small side, and so the resulting loss of
dimension
may be unacceptable.

John Schmitt