A year ago I had a slow leak from a toilet upstairs which damaged the plaster cornice in a corner of my sitting room, this was repaired by using plaster and making a metal template and drawing this along the wet plaster to replicate the pattern. This was left to dry out for about a year and I was about to redecorate the area when I had a huge leak from the pipe above, which ran down through a plaster ceiling rose and wetting all the cornicing around the room. I had a drying company in through the insurance. Now everything is dry, the area previously repaired has a powdery surface, some salts coming through. How can I stabilise the surface ready to repaint, to harden it. I dont want to start raking at it as the plaster parts could easily come off again. By the way the insurance won't pay for any of this, they will only pay for stain block on the plaster cornice and decoration of the cornice and rose.
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Repairing plaster cornice in Edwardian house
- Thread starter starfish
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Gervase
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- 1,500
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- North London
Much depends on the sort of paint you intend putting over it, but a brush down and then a coat of PVA diluted 1:1 with water will stabilise the surface for any modern alkyd paint or a distemper.
You could also stabilise it by painting the surface with two or three coats of very dilute limewash (almost to the point of being lime water) and letting it carbonate, but the resulting surface won't be suitable for oil-bound or modern alkyd paints.
You could also stabilise it by painting the surface with two or three coats of very dilute limewash (almost to the point of being lime water) and letting it carbonate, but the resulting surface won't be suitable for oil-bound or modern alkyd paints.
Gervase
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- 1,500
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- North London
Dilute PVA will be fine. You can get it in five-litre containers from any builders' merchant, but for your cornice you could see if you can get a smaller quantity - one litre mixed one-to-one with water would probably be more than enough.
I have prepared the plaster surface (old cornice but repaired with pink plaster) with PVA mixed with water, as i was putting it on part of the surface was coming off but i carried on and just brushed of the parts that were coming off. I have left this for two days to dry. I have now tried to paint with matt emulsion but it seems to be rolling off a bit.
will this dry?
should I prime with something?
any suggestions
will this dry?
should I prime with something?
any suggestions
alice
Member
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- 247
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- 1 foot in Texas, 1 foot in Suffolk
If you just painted over the plaster with a water based paint, it might have soaked up more liquid than you think, especially if it was wet earlier from the leak. When I had sculpture students working in plaster (when I first came to the school I'm working at now, I taught sculpture and pottery/ceramics/glass), we would have problems with paint rolling off plaster surfaces, and it was always because the plaster was wet. Does it feel cool to the touch? If it feels at all cool, it has water in it and you should let it dry out more. Truely dry plaster feels warm and feels light for it's bulk.
However, once plaster has set, if it gets soaked again it will start to break down and when it dries out again it doesn't go back to a hard state. It's a chemical reaction between the water and the calcium carbonate in the plaster that sets it (which is why it gets hot when it sets); it's not a drying out process and that chemical reaction doesn't reoccur when it's set, dries, gets wet again and redries. It will turn to a powder and that might be what's happening with your cornice.
When the plaster starts to powder, you can try spraying it with a clear matte acrylic topcoat or varnish. Sometimes that will set the surface enough to allow it to accept paint...but first make sure the plaster is absolutely dry.
alice
However, once plaster has set, if it gets soaked again it will start to break down and when it dries out again it doesn't go back to a hard state. It's a chemical reaction between the water and the calcium carbonate in the plaster that sets it (which is why it gets hot when it sets); it's not a drying out process and that chemical reaction doesn't reoccur when it's set, dries, gets wet again and redries. It will turn to a powder and that might be what's happening with your cornice.
When the plaster starts to powder, you can try spraying it with a clear matte acrylic topcoat or varnish. Sometimes that will set the surface enough to allow it to accept paint...but first make sure the plaster is absolutely dry.
alice