John Schmitt wrote:
Kevin Brady wrote:

[wants to skim semi-external loo]

Internal wall is brickwork with a light coat of white emulsion over
(very light coat in places, you can see brick through the brush
strokes). The ceiling is a in-situ concrete, with tiny ridges where
the forming timbers were removed. The ceiling is similarly treated
to a watery emuslion finish.

Ideally I would like to give both surfaces a skimcoat of (bonding?)
plaster. Do I first need to remove the watery paint from the
bricks/concrete? Is bonding plaster the best for skimming (2 coats
max)?

The paint must go. Guessing at the age of the property, it is probable
that under the emulsion is distemper, a composition of powdered chalk
and animal glue. Even if emulsion is the only paint, it is still
likely to compromise the bond. Use an angle grinder with a cup-shaped
wire brush attachment, eye protection, essential (they tend to shed
wires), to remove the paint. The ceiling will need a once-over too as
there may be release oil on the surface. Give it all a coat of PVA to
promote adhesion and reduce suction (Giving you longer to work the
plaster).

Probably it will be cheaper to buy a cheap angle grinder than to hire.
While the cheapo tools are not as well made, for the average
houseowner, they serve the purpose.

Bonding plaster is normally the substrate, finish plaster is the final
layer to provide a smooth surface.

John Schmitt

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.
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.