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?

Are you a qualified plasterer?

No, but I have seen the results of self-made plasterers. Very clearly
some of them failed to read the manual. Unfortunately, the building
trade tends to attract flibbertigibbets and ne'er-do-wells.

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.

The fact remains that if the paint is not well bonded to the plaster, it
will act as a release coating. Depending on the paint, it might even be
water soluble, with predictable results. Relying on the (poor in
building adhesive terms) adhesion of paint is an excellent way to
produce a job which will fail.

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.

The differences between emulsion paints are rather greater. As this
paint job appears to be a makeover, it is reasonable to assume that an
extremely cheap, low quality product was used.

John Schmitt