In article ,
"Kevin Brady" writes:
I have a downstairs loo in a side building attached to the house, which I'm
doing up.

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)?

Firstly, are the walls or roof damp as is likely if it's just a
single skin wall? If so, you are going to require some special
measures as the damp will wreck gypsom plaster. I'll assume not.

Unless the paint is flaking off or washes off easily with water,
you shouldn't need to remove it. I would seal the surface by
painting it with a 1:5 PVA:water solution though.

Then you need to apply a scratch (base) coat plaster. Bonding
coat is good for this -- as the name implies, it sticks to things
very well. The scratch coat should give you a flat vertical surface,
but doesn't polish up and should be left rough for good keying of
the finish (top) coat. Use Multifinish plaster for the finish coat.
You normally do two coats of finish coat, ideally applied when the
coat underneath has set (or almost set), but is still wet. If you
take a break from the job and the coat underneath has dried out,
prime it with two coats of dilute PVA, first at 1:5 and allowed to
dry out, and then second at 1:3 and allowed to dry to being tacky.

--
Andrew Gabriel