Do I need to ‘seal’ old plaster (as I would new plaster) before I paint it?

The background to this is I took the coving off in our little bedroom which pulled patched of paint off the walls revealing the 25 year old plaster underneath. I then discovered that the rest of the paint on the walls could be stripped off easily with a putty knife so now I have bare plaster walls that are ready to paint. So I’m wondering if I can get away with just a top coat?

Thanks in advance!

DAVE

I have just sealed new plaster with watered down emulsion. I was told this is more than adequate. For the little time it takes, I’d do the same on the old plaster.

Dont bother with that sealing stuff. Its the same effort as painting anyway.

the rest of the paint on the walls could be stripped off easily

Probably because it wasn’t sealed.

Watered down trade emulsion (slapped on with a big ol’ brush) should do it as Dutch Richard says.

EDIT: Whatever you do, don’t listen to him ?

Oi if it involves smashing and breaking things, I’m your man for advice 8)

The plaster will be very porous so it will suck the water out of the paint and might not dry and adhere to the wall properly. That’s why you use watered down emulsion to reduce the plasters porosity – 25% water is about right.

As per teef’s advice, the emulsion needs to penetrate the porous plaster,
instead of lying on the top

Dont bother with that sealing stuff. Its the same effort as painting anyway.

No, its not! I painted my newly plastered loft room when I was a kid, I asked my Dad about sealing the plaster but he said no. So there I was, six coats later, still being able to see the plaster through the paint. And being a loft room, the walls were at 45 degrees and you have to paint the ceiling as well. Nearly killed me.

is there a damp problem in the room?
no matter how poorly sealed the walls were before there is no way that all the paint should come off leaving bare plaster.
if you are going to give it a mist coat then do it with a roller as ‘a big ol brush’ 🙂 will leave loads of brush marks that you will never get rid of no matter how many coats you give it.
if it was me i would line the walls and paint over it – proper job

Sealing the plaster with thinned emulsion is for the paint to adhere, not to help coverage.
No need for lining paper if the plaster is clean

fair enough but the plaster’s not going to be clean after scraping 25 year old paint off the walls

A big ol brush will not leave brush marks if the emulsion is watered down. It will however give better penetration into the plaster. Or use a roller. Whatever. They’re your walls. 🙂

op states bare plaster walls, i take it the paint come off in sheets leaving the plaster free from all paint, in which case shouldnt need lined.
But yes i agree if theres traces of paint left it will look bloody awfull without lining.

Thanks for all the advise!

Fingers crossed the snow holds off long enough for me to get to the DIY shop this weekend!

DAVE

Hora in completely useless and inaccurate advice shocker.

I had the same issue as the OP in the bathroom. Previous owners clearly didn’t use a mist coat and the paint scraped off the walls like steamed wallpaper. Filled in any big gouges from the scraper and then sanded & wiped down the whole wall. 2 coats of mist coat (75/25 for the first and 90/10 for the second) and then standard emulsion and it’s bonded fine.

Although, in retrospect, I wish I’d have filled & sanded the bare plaster a little better, or perhaps used a coat of that “filling paint”. There are a few small scraper marks that you couldn’t see on the bare plaster, but do show up once painted.

Go to your local trade paint center and buy the stuff thats been developed for plastered walls. Simples.

I’ve just sealed some plaster using some of the B&Q sealer (instead of watering down emulsion like I usually do). Three things I like about it…

If the emulsion is non-drip it’s a pain in the ass to dilute, you don’t have to do this with sealer

It’s easier to wash out of brushes

It’s colourless, so if you overpaint or splash, you can’t see it.

I can’t remember what it cost, was not expensive, and personally I thought it was a bit easier.

PVA watered down 4:1, apply with a roller, it’ll take 15 minutes, stabilise any dust, (which will stop new paint adhering properly) and kill any suction, (which might save you a couple of coats). You can get a 2.5L tub in B&Q for about £6 that’ll more than cover a standard size room.

For such a small, inexpensive precaution and considering the existing paint has failed, it’s daft not to.

Wouldnt use pva, many plasterers advise this and its bad advice imho.
Pva forms a skin on the surface of the plaster just like it does on your fingers-skin this then can peel off when you get a build up of paint in the future.
A mist coat of the colour paint your going to use is the correct procedure.