John Schmitt wrote:
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.


Are you a qualified plasterer?

I am not, but I have been plastering for the past 8 years and have gained a
lot of knowledge and advice from good plasterers.
Don't take this the wrong way but I think you underestimate the powerful
suction&adhesion of todays plasters.
If I thought for one minute that the OP would not get a sound fix to the
walls then I would have gone the way you described.
Theres a difference between emulsion on brick and plaster.