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A few weeks ago I asked for advice on this bubbling paint and you kindly suggested it was a lime reaction to vinyl Matt emulsion

So I stripped back the paint yesterday to apply a coat of Zinsser Guardz and this is what lies behind the blistered paint. There seems to be a darker spot within the plaster. Does this confirm the view above that this is a lime reaction with the emulsion in this particular patch?

That looks like a plasterboard wall clout nail head. Scrape the spot and see if it is metal below, although it is unusual, it could have caused a reaction. Thinking outside the box, I wonder if the galvanised finish of the nail was damaged, and some rust spots caused the reaction. Check it further and see what you find.

DWD