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

Uncle-C wrote:
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

Why?


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.


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.



cheers

uc

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

Plaster doesn't stick to PVA very well.

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...

--
Tim Watts

This space intentionally left blank...

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

Plaster doesn't stick to PVA very well.

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...

Plaster sticks remarkably well to sealed surfaces, especially plastic
buckets

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

Uncle-C wrote:
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

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.

http://www.toolstation.com/documents...?searchstr=sbr

Dave Osborne wrote:
Uncle-C wrote:
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

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.

http://www.toolstation.com/documents...?searchstr=sbr

Blimey that would be overkill. All pva does is soak into the surface and
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.

On Sat, 07 Nov 2009 13:47:07 +0000, Stuart Noble wrote:

Dave Osborne wrote:
Uncle-C wrote:
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

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.

http://www.toolstation.com/documents...?searchstr=sbr

Blimey that would be overkill. All pva does is soak into the surface and
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.

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

On 7 Nov, 17:25, Steve wrote:

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.

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

On 7 Nov, 18:13, geraldthehamster wrote:
On 7 Nov, 17:25, Steve wrote:

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.

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

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

geraldthehamster
wibbled on Saturday 07 November 2009 18:13

On 7 Nov, 17:25, Steve wrote:

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.

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

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

--
Tim Watts

This space intentionally left blank...