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.

In an earlier contribution to this discussion,
Milleniumaire wrote:

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.

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!)
--
Cheers,
Roger
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Roger Mills wrote:
In an earlier contribution to this discussion,
Milleniumaire wrote:

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.

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!)

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.

Owain wrote:
Milleniumaire wrote:
We have a sun room attached to the kitchen which has a sloping tiled
roof. .... 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.

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


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