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We've recently had our spare bedroom ceiling plastered, and there was a loosish bit of plasterboard that was secured by the plasterer before skimming the entire ceiling.

After painting it I observed a strange brown patch following the line of the underlying plasterboard. The only thing I have ever seen that is similar to this is the brown marks that appear on painted wood where a knotting solution hasn't been applied, or from the corrosion from underlying rusted nails.

The spot itself is directly below an attic water tank which looks like it may have leaked in the past (when the previous owners still lived here). However a thorough examination of the area revealed it to be bone dry (as was the area before plastering, and is now).

Does, anyone have any thoughts as to what might be causing this and steps I should take to remedy it?

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A photo may help with this. – BMitch May 29 '11 at 21:45
@B Mitch: I'll see if I can get one. – James Wiseman May 31 '11 at 8:17
up vote 2 down vote accepted

I had this exact situation when flashing around a chimney leaked and I had some water spots on the ceiling by the fireplace. The stains from the water can still come through even after the drywall is dry, so you need to apply a sealer and repaint. I used Kilz; you should be able to find a similar product if that's not available where you live.

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Thanks, Niall. Will give your suggestion a try. I live in the UK, so will post back some similar products here when i find out. – James Wiseman May 31 '11 at 8:20
In the end I used a knotting solution I had to hand. No dirty brown stains now. – James Wiseman Jun 23 '11 at 14:23

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