Painting damp walls

23 May 2007
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United Kingdom
In the living room of my son's middle floor tenement flat, damp appeared in the corner of an internal wall, external wall and ceiling. Turned out that upstairs had a rising mains leak which was eventually fixed late 2022.

However, both walls about 1m each in from the corner were damaged. The attached picture shows some of the streaking and lines on one of the walls. We understand there was cavity wall insulation inserted in the light purple wall well before my son bought the flat. The leaking was a sort of guey substance which then dried with a crust. We let the walls dry then scraped some parts back to the very original plaster coat and some parts just sanded. They seemed to be dry and I then painted two coats of Zinsser bin allowing it to dry in between coats.

A few days later some bubbling appeared then we got some yellowish drips on both walls which appeared to be coming from the larger bubbles. Depressing the smaller bubbles brought out some small amounts of fluid. There are bubbles appearing all over the affected areas.

We're really not sure what to do with the walls. I've never had this sort of issue with zinsser bin before. I'm guessing that perhaps the walls weren't fully dry despite leaving them for a couple of months and sanding them. The cause of the leak was fixed months ago. I'm now in the process of scraping everything back to plaster where possible.

Wondered if anyone has any ideas please ?
Would a damp proof paint work if painted on a surface which appears dry but perhaps isn't ?
Which dampproof paint would recommend ?


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Still have water in wall I think.
One coat of BIN with a 4inch roller (I'd use form roller) and give it an hour then on with emulsion. That should of been fine.
Damp paint works in old properties but I wouldn't. It's corner cutting product.
Need to remove paint and dry it out.
Put summer fan on for couple hours a day blowing that way for a week on the plaster
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Could it also be that the affected plaster is contaminated?
Thanks. The walls have been fine until now so we'd discounted contamination. However, it's possible there is some which has only appeared due to the water damage from above.
It sounds like the cavity insulation is still wet.

I would be inclined to drill a series of 10mm holes in the wall to allow air in and moisture out but I can understand why you might not want to do that.

When BIN is applied over wet walls it will fail. It prevents the moisture from escaping. The water pretty much pushed the zinsser away.

If you don't want to drill holes, you could use a sander to remove all of the paint in the damp area and apply a low latex paint (eg Dulux Trade Supermatt, that will not act as a moisture barrier and will allow the moisture to evaporate off. It may still take a number of months to dry out though.

Zinsser do make a cellar paint that can hold water back, but you need to let the water escape.

Edit- BTW when working with BIN, use a tub with household ammonia in it to clean your brushes/etc. It is an alkali and completely breaks down the shellac in the BIN. I use expensive brushes when working with BIN, safe in the knowledge that they will clean perfectly. You can then pour the ammonia back in to the bottle for the next time.

It does stink though.
Try clean spirit Vs ammonia.
After 48 wash out under tap water
No smell lovely to use
Try clean spirit Vs ammonia.
After 48 wash out under tap water
No smell lovely to use

What is clean spirit?

TBH, I don't mind the smell of ammonia in small doses. These days, when using BIN, I clean the brush or roller immediately. I only have to deal with the smell for a few minutes.

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