PVA and plastering question.

[Uncle-C]

Hi, I've just rid a small wall of old plaster to reveal some old brick , some partially painted. I've scrapped, smoothed and sanded to try to get rid of any excess paint and now have a reasonable conditioned wall to plaster. I would like to PVA the wall first but there seems ambiguity over how long I should leave the PVA on the wall prior plastering. As the main purpose should be as a primer should I leave to PVA on for an hour or so ? I have heard people say leave it overnight but I'm sure this is wrong. Could someone kindly confirm.

cheers

uc

[The Natural Philosopher]

Why?

I have no idea why anyone would PVA a wall prior to plastering, unless it was loss and crumbly and non structural.

Plaster doesn't stick to PVA very well.

[Tim W]

The Natural Philosopher wibbled on Saturday 07 November 2009 11:05

The plasterer I had used PVA, both on the friable original undercoat plaster, and over painted bits.

I've done the same on the room I plastered, with good results. I thought it was the standard way these days...

[Stuart Noble]

Plaster sticks remarkably well to sealed surfaces, especially plastic buckets

[Uncle-C]

Thanks gents. The question still remains - leave the PVA on for one hour or overnight ? The surface is smooth but is prone to slight crumble.

cheers uc

[Dave Osborne]

You only need to leave it until it's dry (touch dry or slightly tacky). A few hours in a warm room is fine.

If you want a higher performance solution (i.e. waterproof and better adhesion) then use SBR (styrene butadiene rubber) in lieu of PVA. It's much more expensive, though.

[Stuart Noble]

occupy the space that the water in your mix otherwise would (thus preventing it from curing properly). IME it makes no difference whether the pva is dry, tacky, wet, or even mixed in with the plaster. The way emulsions, like pva, work is that the non water soluble molecules band together and push the water on to the surface where it evaporates more quickly. In theory the pva content should always end up furthest from the surface, even if it's the last thing to be added to the mix.

[Steve]

Yes, the PVA helps to seal the wall by filling the spaces as Stuart says. It also acts as a bonding agent.

The way I learnt was to PVA the whole area first with *slightly* diluted PVA, let it go tacky or even dry then just before plastering do it again. When tacky, it is time to plaster.

It is not that critical and sometimes I just do it the once. As long as the plaster is going on to "tacky" PVA, it seems to bond well.

HTH

SteveE

[geraldthehamster]

I had a plasterer reskim my hall recently, which was tatty and friable in places, and painted in others. I PVA'd it the day before, at his request, then after that had dried, he PVA it again and plastered while still tacky. I don't know how long he left the second coat for, exactly, but I don't think it was long.

Cheers Richard

[JimK]

IMHO the *last* coat of slightly dilute pva should be "invisible"(i.e. no droplets or beads showing) aka tacky before applying the plaster - if wet it can mix with the plaster (skim coat) and cause some local blemishes...

JimK

[Tim W]

geraldthehamster wibbled on Saturday 07 November 2009 18:13

Interestingly, that was exactly the request my chap made. It was also the way I did one room myself.