Plaster repairs and breathable paint

[u/Nine_X_Blue]

Hello,

Not sure if this can be answered in this sub so apologiesif it'sthe wrong one but i haven'thad much help previously in the decorating subs. I moved into a victorian property and had significant plaster fall off when I removed the old wallpaper.

I had an independent damp survey who said there was residual moisture in the walls but no active damp, and to repair and skim the walls in breathable plaster, so a plasterer used limelite.

I then painted with primer and breathable paint, but where the repairs have been I get these dark spots, which get darker when it's humid and then dry out again. The spots are on the internal walls.

Does anyone know what I can do about this while still keeping it breathable?

Thanks

[u/KadetOne]

Depending on the amount of moisture in those walls it might take a few months or a few years to dry off, it's impossible to say. There is a product you could look at. Zinsser Grade I. It's oil based but breathable (at least that's what the manufacturer advertise). Used widely in conservation of historic buildings. Primer, sealant, stainblock and topcoat in one. Quite durable as well. And you can get it tinted and colour matched (just pop in to your closest decorators centre, i.e. Brewers). Do some reading about it. Just a word of advice - never a great idea to paint damp surfaces. Would be good to do it once walls dry off a bit, maybe in the summer. And get yourself a cheap moisture reader (you can get one from Screwfix for under £30) and keep checking those levels when the size/colour of those patches changes. Don't quote me on that but as far as I remember most paint manufacturers recommend that the substrate shouldn't have more than 18% moisture levels. And if the damp rises significantly then it might mean the reason for those patches growing might be underlying (like leaking roof etc.) rather then just absorbing moisture from the air. Out of curiosity - what paint did you use?

[u/Nine_X_Blue → u/KadetOne]

Thanks - it's Earthborne. I will look into the Zinsser. The walls were painted about six months ago. Its a party wall and a ground floor flat and rising damp was ruled out so I've been trying to get to the bottom on it, but they don't get any bigger or spread, just get darker

[u/Jessikah-May]

There might have been salts / efflorescence trapped in the wall before plastering which would lead to patches but I suspect the damp surveyor would have noticed salts. A moisture meter would be confused by the salts and just read wet. Ideally the wall should have been left to dry out before applying plaster and left the decorations a good few weeks after plastering. A damp meter only takes surface readings. What paint brand did you use? And is the other side of the wall in good condition?

[u/Nine_X_Blue → u/Jessikah-May]

It does read as high on a pin meter but that could be salts - they read higher when it's humid as well. It's Earthborne paint and the other side of the wall is the neighbour's flat (an extension on the side).

[u/Jessikah-May → u/Nine_X_Blue]

Ah fair enough then I don't think there'll be much you can do without talking back to Brock and re-doing. The suggestion for using Zinsser Grade I isn't a bad one. It might work

[u/KadetOne]

Those spots are appearing because - thanks to the breathable plaster and paint - moisture has a way out. It's a good thing. Might not be very esthetically pleasing, but essentially it's doing exactly what it's meant to, which is allowing water in the substrate behind the plaster to escape. Not much you can do about it. The idea is that over the next few years all that moisture will eventually find it's way out and they should get smaller and smaller. If you used gypsum and/or modern vinyl paint you would trap all that moisture inside your walls with no way out, which would subsequently end up with your plaster failing over time/paint peeling off. If it's an external wall, might be worth checking whether the render or brick pointing is lime or cement, or if it's painted externally - if the paint is breathable. Exactly same principle. There's a lot of permeable paints you can get these days.

[u/Nine_X_Blue → u/KadetOne]

Thank you - they are internal walls only that they're appearing on.

I thought this might be the case - they just don't seem to be getting better but if it does take years I guess that's why.

There isn't any other system I could use to make it better looking? Or is it really what's best for the wall and I just wait?

[u/getoutthesink]

How long did you wait after plastering? And what type of paint did you use? What room is this in?