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