nailheads under plaster showing up dirty on paint

[tone earwaker]

plaster board with insulation was fitted to our bathroom cieling and walls with flatheaded nails. It was plastered and when dry painted with silk emulsion. Not long after grey spots formed over the nail heads that could not fully be wiped off. They are now dark grey.
What can I do to prevent this happening again when I redecorate.

The bathroom is well ventilated with an automatic humidity sensitive heath exchange unit.

T.E.

[chris_on_tour2002]

cut away the plaster and expose the nail head plus a bit more. using pva glue, stick a piece of silver foil over the nail head. once dry refill, sand and paint. that should stop it for good. repainting wont help the stain needs to be stopped.

[tone earwaker]

Thank you Chris for your reply.

Do you think it possible that the difference in electrical charge, or temperature between the area over the nailhead and the plaster generally could attract dirt and so course the grey patches?

I have approx 200 nailheads so I am not relishing your suggestion of uncovering them all, especially as most are on the ceiling.

Was there anything our builder or plastere should have done to prevent this happening.

With thanks

Tone.

[chris_on_tour2002]

not sure about the electrical charge or temp diff... most likely just a reaction between the nail head and the wet plaster when it was applied. if the builders have used the correct nails they are galvanised and dont normally react like that.

would be a pain to treat the whole ceiling as i suggested unless you fancy baco-foiling the whole lot. use a stain block instead, aquaseal for example. that should stop it.

[tone earwaker]

Thank you Chris,

I will try aquaseal hoping it works.

Cheers

Tone