Just to give a brief on what's going on here. I currently live in a basement flat which is more damp than it is home at the moment. Obviously this being a basement I'd expect such, and there's a few mouldy patches forming in corners which I think are just caused by inadequate air circulation.
The thing I'm posting about is that a number of the interior walls (Right in the centre of the property) seem to be experiencing damp issues from the ground to about 50cm up. The paint is coming off of the walls, there's fluffy white deposits on the surfaces too. I believe this was caused by a leak in the flat above, which we were unaware of until one day it literally started dripping onto the bed. The leak was fixed about 6 months ago, when we and the neighbours became aware of it. Upon inspection the walls themselves have dried out completely.
The plasterboard on the walls seems to be fine, it's dry, I see no damp or mould patches on it, it's just the paint on the surface that seems to be affected.
My question is am I likely to need to replaster these areas anyway, or will just replacing the paint suffice?
This is infact salt coming throughout the paint from the plaster behind caused by the leak. What you need to do is scrape back any dusty plaster/paint and treat with Alcli Resistant Primer. You can pick this up in 1 litre tins in dulux trade suppliers (like B&Q). One coat with a cheap brush (ARP is solvent based and results in a dirty brush when using it) that can be thrown away afterwards.
Once this is dry you can repair the damage with interior filler and make good.
This should stop the salt coming back through. Failure to prime the plaster will likely cause the filler or paint to peel back off.
It will not be only due to a leak if there are salt deposits on the internal plasterwork. Fresh water does not contain salts which would deposit onto the plasterwork during evaporation. Ground water such as low level rising dampness and penetrating dampness (If the wall was retaining) contains salts which you are seeing now.
Imagine boiling a pan of salty water on the hob until all the water evaporates, you will be left with salt crystals at the bottom. Rising dampness works in the same way theoretically. The salty ground water is absorbed by the masonry, when the masonry reaches saturation point, the dampness tracks out to other areas and substrates such as your plaster. The surface of the plaster is the first thing to dry out due to natural evaporation which leaves the salt deposits behind. This is called efflorescence.
If you do not treat this area, these salts absorb further moisture from the atmosphere and continue to make the wall damp.
One "ALMOST" sure way of telling wether a damp spot is rising/penetrating from ground water or fresh water or even condensation is due to the presence of mould. In all the years I have been in the industry, you very rarely see mould in areas of rising dampness due to common moulds not being able to survive on a salt rich substrate.
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