Yellowing plasterboard ceiling

[Milleniumaire]

We have a sun room attached to the kitchen which has a sloping tiled roof. I noticed after the heavy rains earlier this summer that In one corner of the room, just below where the sloping tiled roof meets the outside wall the plasterboard on the ceiling was damp and started to discolour. I hopefully fixed the problem outside by re-mortering the flashing (this was lose) and filling any other cracks etc I could find. It's difficult to say if this has definately fixed the problem as the ceiling is still discoloured, although the plaster/plasterboard is dry and solid i.e. my fingers will not break it. I decided to tidy it up by repainting using white emulsion, however the paint will not cover over the yellowing patches, no matter how much I put on. It's almost as though the yellow stain in the plasterboard just seeps through each layer of new emulsion.

Has anyone any idea why this is happening and what I can to to stop it? I assumed the plasterboard was okay as it is dry and solid, but now I'm not so sure. The area affected is about a foot square and is mainly in the corner of the room.

Thanks for any advice.

[Roger Mills]

Emulsion paint just won't cover such stains - you need an oil-based paint. You can get some expensive fancy paint just for that purpose - but oil-based white undercoat is just as good. Once that has dried, you can then emulsion over it (or, literally, under it!)

[Owain]

You could try some 'stain block' undercoat.

If 'it' still comes through that then 'it' is probably damp. Cutting out a few sq ft of plasterboard and replacing, and taping the join, should not be a too major job.

Owain

[The Natural Philosopher]

MM. You may try a white acrylic wood primer..that is pretty good also and not so hard to get emulsion to go on top of.

[Stuart Noble]

IME the stain will come through any water based paint. Take Roger's advice.