What to put on bare plaster before paint?

[AnnOnymity]

I've been stripping wallpaper and want to paint the walls underneath. Some of the bare plaster is 12 years old (we know because the plasterer wrote his name and the date on the wall!), but one wall has very old plaster (this bit of the house is 270 years old).

Do I put plaster sealant on first. Or diluted paint (50:50 water and paint)? Or something else?

You can put diluted paint on but do NOT put pva on. It's an old method but actually doesn't allow the plaster to breathe and so causes damp issues later on.

Ah, thank you. The walls are old and rendered on the outside so need to be able to breathe on the inside.

Diluted paint will be enough (first coat 1 part water to 3 paint) and then another thinned coat (1 part water to up to 5 parts paint) according to theDulux Trade site (we have the plasterer in today and I was looking it up this morning!). Just use bog standard white emulsion (it will probably advise on the tin)

I wouldn't bother with plaster sealant if you've used thinned emulsion.

If the walls weren't pained before they were wall papered you might have problems. The wall paper paste could have soaked in and might stop the paint sticking on correctly.

I would try it with a diluted mix of whatever you have in the house (matt emulsion not silk) first to see what happens without spending any money.

Fingers crossed!

Thank you everyone. That's really helpful.

You might want to sugar soap the walls first to try and remove any traces of wallpaper paste etc. and then do the paint/water mix. Not sure though as am no expert I just always do it when painting onto wall paper or over paint.

to get old wallpaper paste off, spray the wall with warm water from a garden sprayer, and scrape the resultant slime off using a broad metal scraper, and wipe it off on your sleeve a rag. It is fairly easy.

To mist-coat the walls, use a non-vinyl matt emulsion thinned like milk with water. Dulux Trade Supermatt is very suitable. It should be so thin that it soaks straight into the wall leaving it looking dry and unpainted. Give it another coat. When the wall stops sucking your big brush dry it is ready. 25% to 50% water is not too much for mist coats. Two or three thin coats are better than one thick. Then you can give it a finish coat of Supermatt to equalise the colour and texture before you later use your more expensive finish paint.

White is cheapest but if you are doing the whole house and want it to look decent before you get round to using some fashionable brand of overpriced paint, Supermatt can be mixed to order in a range of 150 colours at a Dulux centre or some big DIY sheds or paint shops.

Non-vinyl matt allows new or damp plaster to continue drying out, because it is not moisture-proof. It forms a good base for subsequent paints.