Dark patches on internal chimney wall

Skint_yet_Again

I had a couple of dark patches so the wall was taken back to brick. No signs of damp, no rotten skirting, no signs of water through concrete floor. Chimney is not used and ventilated. Treated for salts and then recovered left to dry. Painted with zinzer bin several coats per instructions then Matt emulsion. Used a dehumidifier in the winter in case of condensation as it’s a cold wall. Dark patches have reappeared in places. Any help or advice would be appreciated. Sorry I am not very good at diy but I have been reading that zinzer cover stain may be better as it is oil based?

stuart45

Have you got a few photos to stick on?

Skint_yet_Again

It’s a very dark corner and my camera isn’t great but hopefully you can see it on these pics. It’s more noticeable in person 

daveyjp

Photo of front woukd be helpful to look at adequacy of any ventilation as I suspect it isn't doing a good enough job.

Skint_yet_Again

Think I will try zinser cover stain or polycel spray as both seem to be oil based stain cover. I have a new tin of emulsion paint so hopefully the stain will not come through 

FreeBear

Skint_yet_Again said:

Think I will try zinser cover stain or polycel spray as both seem to be oil based stain cover.

With the Zinser stain killer, it is shellac based. You'll need an alcohol to clean your brushes rather than something like white spirit. Methylated spirits is as good as anything.

Skint_yet_Again

Thanks Freebear. Their website shows “bin”is shellac based but “cover stain” and “bullseye” are oil based. I was thinking of using their “cover stain” spray or similar so no need for brushes. 

stuart45

Hygroscopic salts can be quite difficult to stop from finding their way to the surface on chimney breasts. When you block them it sometimes works, and others they turn up in another spot. 
It sometimes happens that people have to fix 2x1 battenss with 4 inch DPC behind them, and plasterboard, or insulated plasterboard on top.

elle_may

Had this,  found chimney needed repointing, not had it since. 

Skint_yet_Again

Thanks elle_may. My roof, chimney & flashing were all done 2 years ago when I moved in. No signs of any wet or damp inside the chimney. It previously had a back boiler at the back and gas fire in front, both have been removed. 

It looks like may need treating for salts again and using a different sealer. Can anyone recommend an oil based sealer that I can emulsion over?