Is it really necessary to prime bare plaster before painting?

[MaryMotherOfCheeses]

I know bare plaster will soak up paint, but if I'm painting a ceiling white, can't I just do two coats of white paint.

It seems odd to do the 50/50 dilute the first coat thing, when you've got to do a second anyway.

Plaster is very dry and will just suck up the first coat so it won't go very far. If you dilute it the paint will cover better and will go further so it will end up costing u
you less in paint.

ah that makes more sense about it costing less.

but then I have to buy a bucket....

Swings and roundabouts perhaps!

Builders told me to put a first coat of very watery paint on first. Soaks up the water and it grips better.

if you don't use a mist coat then you run the risk of the paint just peeling off - not a good look!

You need to do a mist coat plus two top coats. If you skip the mist coat and just did two thick coats of paint, it would sit on top of the plaster like a film and eventually start to peel off. The watery mist coat sinks into the surface of the plaster, leaving behind a good surface for your top coats to stick to. You have to use a good quality non-vinyl matt emulsion for the mist coat (I would use it for the top coats too, as it lets the new plaster breath). Don't use a one-coat or a satin as these form a film and will peel. Wickes trade matt is very good, and cheap! It's only £20 for a huge 15L tub which would probably do your mist coat and your top coats, but they do smaller tubs if you don't need as much. And they do a bucket for 99p

Whatever you do, don't use PVA on new plaster. Someone always suggests it when people mention new plaster, but it's just one of those myths and causes loads of problems.