Why do you own to river down emulsion when you put it on plaster?
Answers: Only the first coat on new plaster.
There is no staunch primer for plaster. The way you prime is by thinning the first coat down so it can soak into the plaster and draw from a grip.
If you don't do it the unthinned paint tends to lay resting on the plaster which is a weaker grip, it can blister immediately and chip rotten later.
A worthy decorator will also apply the first coat with a brush to achieve under any dust residue on the plaster.
That's why it is better to take on or learn from qualified time served decorators who go to college rather than brush hand, self-taught and builders.
I always wet down the first coat but only by a small amount, or the paint will drip everywhere. Helps it to spread easier and save money on paint because less is used. You other need two coats of paint whichever method you use so it's your choice. I approaching to use paint pads on walls cos it's much faster to do the work.