In article ,
"Lorna" writes:
Just had an extension built and the internals have been plastered. Some is
dark brown, some light. Assume that the dark means it's wet so I'm waiting
for it to go light before we paint. Two things :
1) People have told us to add water to the paint. Why should this be done?

Thinning the paint helps carry it into the surface of the plaster
and make it stick.

What will happen if we dont?

The plaster will suck the water/solvent straight out of the paint
without drawing any of the body of the paint into the plaster surface,
and the paint won't stick to the wall. Often this causes the first
coat to come off when you try to apply a second coat, and/or all the
layers to wear or even fall off the wall at a later date.

How much water should be added?

It will normally say on the tin -- something like 10%. If the plaster
is very smooth and polished, add a bit more than this.

2) Can we paint straight onto the plaster or do we need to rough it up to
make the paint stick?

This depends how polished it is. You can polish up plaster like a sheet
of glass, but then it's difficult to make the paint stick. If there are
any hightly polished areas, you might want to take the shine off with
sandpaper first.

--
Andrew Gabriel