I've been asked to sort out a ceiling that has brown stains from there having been a leak from the room above.
My experience of stains like this is that however many coats of emulsion you apply it still ends up brown. I remember there being an aerosol called Stain Block or some such - is it still available and does it work?
There are a number of aerosol preparations, I used two, but have no recollection of the name. I used two because I dumped the first thinking I would never need it again :-) I would guess that they are all of a similar mix.
Standard bedroom ceiling, I noticed a brown patchon the plasterboard around 20years back. Did nothing apart from inspect the loft [Dry]. Patch darkened and spread at around 0.7m^2, I bought my first spray a couple of years after the first sighting.
Job done, cured, for two years then a respray was needed.
Over 10 years it has had around five treatments [10 minute job]
The brown stain is just starting to reapear now.
The return is now more sluggish and only noticeable I am sure because I look for it.
Probably the best solution would be to remove the plasterboard and replace.
I don't regret using the spray, even if the occasional recoat is needed, replacing the plasterboard wouldnt be fun.
Incidentally, the stain looks like a damp patch, but the loft is bone dry and the wall in contact with the dicoloured ceiling is plasterboard also and isn't stained.
Maybe the stuff was contaminated with something during manufacture.
In my case I was happy. A quick squirt over the years was a small effort considering the result.
there are several things you can use to block stains. Most work most of the time, none work all the time. Undercoat for woodwork, the alkyd solvent sort PVA diluted shellac in solvent shellac flakes dissolved in alcohol commercial stain block paints eggshell paint
I'd seldom recommend sprays, sprays tend to get everywhere and don't keep well. In the worst cases it can be necessary to keep alternating spirit based & water based layers several times, but usually one layer does the job.
Dunno but something made by unibond fixed my stains from a loft tank leak. It was on poly tiles and it seems to have lasted after a couple of coats of emulsion on the ceiling, but sorry cannt recall the product it was some years back and suggested by a bloke in b/q, the stuff dried a kind of crackl hard shell on the times as I recall. Brian
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