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