Hairline Cracks in Newly Applied Plaster


Old 08-21-17, 07:40 AM
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Hairline Cracks in Newly Applied Plaster


I have a house more than 100 years old. The walls are plaster. I'm restoring the house and recently had a large mirror/mantle put in. Turns out the wall is not flat. So in the middle of the wall there's a gap of about a half inch. Above and below the middle the gap is not so pronounced, as the middle of the wall apparently is the most concave.

I decided to plaster the section of the wall adjacent to the mantle to make it flat. Two days ago, I applied a basecoat of Structo Lite. Before starting I wet the wall with a sponge. (The wall was scrapped and painted about a year ago).

The bulk of the new plaster is still brown or drying. The edges, which were thinner to blend into the rest of the wall, have gone white and in place are showing hairline cracks.

What did I do wrong? Is this because the coat was too thin at the edges? It dried too quickly?

Also, how should I fix the problem? Do i need to scrape and replaster from scratch? Should i simply sand out the cracks at the edge, then apply the top coat of gauging plaster? The part that's still brown appears to be fine.

Some potentially relevant info: I initially applied the basecoat on one side of the mantle then went to work on the other side. After finishing the other side (maybe an hour later) i realized the plaster wasn't even on the first side so i built it up. I'm not sure if this delay in applying plaster may have caused problems.

Also as mentioned i plastered over paint. Not sure if this has something to do with it.

In addition to my questions above, any advice on when to apply the top coat of gauging plaster and lime? The man at the store said 24 hours. But the basecoat still is not dry. Obviously it's been more than 24 hours at this point. If it matters, I'm using Red Top gauging plaster mixed with Red Top lime.
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Old 08-21-17, 02:26 PM
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I probably would have patched it with joint compound instead of plaster. I don't know how new plaster bonds to old plaster. Bonding could be the problem.
Old 08-21-17, 09:43 PM
Join Date: Jul 2003
Location: California
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She I am on a computer rather than this iPad I say more. In the mean time try this. Mix up a straight ng alum said Luton. and wet the plaster wth that. It should set the plaster.
Old 08-22-17, 05:47 AM
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What kind of solution? Seems like you may have lost the battle w your ipad
Old 08-27-17, 05:55 PM
Join Date: Jul 2003
Location: California
Posts: 1,722
I am back at a real computer. I fear enough time had elapsed that my advice is void.
What would be more helpful now is a picture or two and a report on what you have done.
If the plaster has not set and it sounds like it had not it should be pretty easy to scrape it off and start again.
StructoLite is notoriously slow setting it needs some help to set. We can tell you what to do.
Most plaster products do not bond well to paint. If you can scrape it off we can tell you how to help the bond.
If you cant scrape it off then maybe it has set and you can just skim it with some joint compound to cover the cracks and proceed to finish.

Pics might help us advise.

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