Blown Plaster

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Hi

I am trying to decorate a room in our new house which is on the first floor of a 80s double storey extension. I have found a lot of blown Plaster on one wall particularly. The would have been the original external brick wall, I'm guessing is blown because of what it was applied to. There's also a crack running down it, what do people think would the cause be?

I'm guessing a new skim would have to be on the base render after its sealed?

Thanks
 

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That looks like seraphite, I have it in my house. The top coat is very prone to cracking. Unless the seraphite base coat is also blown, it’s a case of patching and sanding or getting a skim done. Buy a cheap moisture detector and make sure you’ve no damp issues
 
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That looks like seraphite, I have it in my house. The top coat is very prone to cracking. Unless the seraphite base coat is also blown, it’s a case of patching and sanding or getting a skim done. Buy a cheap moisture detector and make sure you’ve no damp issues

Thanks, the very course layer in the photo doesn't appear blown, just the plastering on top. I was thinking about testing the moisture too

Has somebody used a steam wallpaper stripper on it?

No, not a far as I know. Potentially many moons ago as I did find tiny pieces of wall paper on the wall. Would the stream also crack the seraphite layer?
 
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Yes, if it is held in one place long enough to make it hot.

In your photo the base coat appears to have stones in it, and to be cracked, so perhaps it is old mortar render, on the wall surface that was previously external and the crack is still moving.

The sirapite skim used to be applied very thinly to a bonding or browning basecoat. I don't know if it was suitable for cement render. It gives a good hard, smooth surface that can be polished up. Now discontinued. I remember a old plasterer telling me that he applied it so thinly that it did not add to the thickness of the wall, it just filled in the pores, shrinkage cracks and imperfections in the backing coat. Modern finish plasters are typically 2mm thick.


Edit
I just looked it up, and it is a "fully hydrated" gypsum plaster, like Keene's cement, which I have heard of but never seen. Possibly something similar is used in restoration work.
 
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Oh, I thought the cement render was being called syraphite, never heard of syraphite before.

I went out and bought polycell plaster repair yesterday. Only concern is the dust from the cement render underneath, I tried to clean in but it seemed never ending

Thanks
 
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You can prime the dusty old render with 1 part PVA glue in 3 parts water. Apply one coat, let it sink in and dry.

When you are ready to replaster, apply another coat and plaster onto it while still tacky.
 
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