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Looks like it's where the filler would have been, so potentially it's just that absorbing moisture (like the canary down the mine), maybe not helped by a metal back box on an exterior wall. I don't want to think it's anything sinister . Sealing over with bin should slow this - think I'd also prefer a ready mix filler over a powder mix
bluehuw
1900 semi detached house, external south west facing first floor corner, this room was plastered and decorated three years ago and now this. Any thoughts on what I need to do? We're likely moving a three year old in here soon, so OH is keen for some action. Cheers.