Pva on plaster before painting

[thomp1983]

Hi all,

We've recently had our living room freshly skimmed and it's all nice and dry.

To try and help out while I'm at work tonight my wife has decided to coat the walls with watered down pva instead of using a mist coat of emulsion like I planned to do tomorrow.

She's adamant it's the right thing to do and she's done it plenty of times before with no issues yet I can't find anything that supports that infact everywhere says definitely do not use pva.

Am I now resigned to all the paint peeling off in the next few weeks and having to do it all again?

Thanks
Chris

 

[foxhole]

Never PVA a wall that's going to be painted.
The paint will just sit on top of the PVA and in time could peel off.
Apply a mist coat of white or magnolia matt emulsion first ....approx 3 parts paint to 1 part water (mix well) and this will soak nicely into the plaster and give you a good base to work on.
Let it dry out thoroughly before applying your topcoats.

 

[sparkwright]

No. No. No. NO. NO. NO!

Do you use PVA on anything you want to paint. Do not use PVA on anything you you want paint to stick to.

(A few good pros can and have used PVA successfully, but generally it is not recommended .)

You do need to prepare yourself for the worst.

What ratio of water and PVA did she use?

 

[thomp1983]

What you've said exactly fits with everything else I've read, is there anything I can do about the fact half the walls are pva'd now apart from make sure it's fully dried out before mist coating it

 

[sparkwright]

Somehow even if the PVA is fully dried out I doubt the emulsion will bond properly.

If you emulsion over that, it will crack, and peel, most likely.

You MAY be able to apply a special primer such as Zinseer over the PVA.

Don't do this until someone else agrees!!

 

[thomp1983]

She says it was about 5:1 water to glue so fairly weak

 

[sparkwright]

I don't know where this PVA thing came from, programmes like Changing Rooms I suppose.

 

[thomp1983]

She's adamant we did the same in the old house, I don't think we did but that paint lasted 5 years with no issues

 

[sparkwright]

Well, you either take a chance - or find a suitable primer.

 

[opps]

The main problem with PVA is that it re-emulsifies when wet (eg when you apply water based paint over it).When rolling, the roller can rip the paint off the surface (read: the PVA off the surface of the plaster). That is because the roller expels air as you push it against the surface and it ends up "sucking" up the substrate. The same kind of thing happens if you don't dilute new paint over fresh plaster.

Additionally, given that it is not porous, the emulsion doesn't have a chance to soak in to the plaster, and in some, but not all, cases it will be less scratch resistant. Each coat of emulsion will take longer to dry given that the water has to evaporate out in to the room.

You may well be OK though. On several occasions I have had to apply emulsion over "waterproof" substrates, it normally only becomes an issue in areas of high humidity- the emulsion can end up crazing but in such environments one should be using paints such as waterbased eggshell anyway.

PVA over new plaster is a bad move, especially if you need to sand any high spots back but you may well get away with it.

I expect to sand newly plastered walls. I do so before I apply the first coat of dilute emulsion. I then find that I have missed some imperfections. I can sand the first coat of paint without worrying about the edges of the paint ripping. In my experience a 5:1 PVA mix does not sand without tearing.

The interwebs is crammed full of people telling you to dilute emulsion by up to 30% or 50% water (or more). Ignore them, follow the advice on the back of the tin of paint. Paints vary in thickness, the manufacturer is best placed to offer advice. BTW the thinner the paint, the more paint spatter you will end up with.