Odd patch after painting

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Hi,

I have this odd patch on my wall. Ive painted the wall 4-5 times and even tried stain stop, but the patch is still visible once the paint is dry. Any ideas how I can fix this?

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OK as a quick check of something, could you [if possible] place a thin bit of tissue against the vent in the wall??? tomorrow is due to be very windy.

Why?? to see if there is an air flow through the wall vent ? if there is some sort of a blockage either behind the grill you can see because of debris falling down the chimney, or there may not be a vented cap on the chimney pot at roof level.

If the tissue is sucked towards the vent on the wall, or indeed blown away then the flue is "relatively" clear and that will eliminate one potential issue.

Sorry no silver bullet fix the possibility of an answer.

Ken.
 
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The tissue was sucked towards the vent

OK that is a positive indication that the old chimney flue is "clear" and there is a positive air flow within the old flue, an air flow keeps the flue clear of any condensation that will form in a blocked of old chimney flue.

Is this problem on a ground or first floor?

What state is the chimney head above the roof line in? + if you can see it does the lead around the chimney look OK?

Having said [as first paragraph above] If the chimney head above the roof is in poor condition, or the lead flashing is in poor condition then no matter how good the air flow inside the old flue water staining can??? occur.

What happens is that rain water can enter the flue, especially if there is no vented chimney pot cap, or?? if the brickwork on the chimney head above the roof is missing mortar, or if rendered missing bits of render Etc. and if the lead flashing is compromised then a lot of rain water gets into the flue.

once rain enters the old flue it mixes with the soot which will be stuck to the sides of the flue, this soot is the result of combustion of fossil fuel that all give of Sulphur, once wet this Sulphur containing soot will give of a mild Sulphuric Acid, this acid attacks the mortar and produces dark marks internally.

Ken.
 

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