Yellow stain on re-skimmed ceiling

[Matthew32]

Hi had my kitchen ceiling skimmed about 5 weeks ago, over the top of the old ceiling which was just in a bad state. It was boarded originally. Everything has been fine, until yesterday, I noticed a yellow stain in one of the corners. Ironically it’s the corner than dried out first after plastering.

When I painted to new plaster I waited until it had throughly dried out, did a list coat of emulsion, left it’s 24 hours and then painted. It’s been fine until now:

There’s no pipes or leak, so I’m wondering if it could be contaminates on the original boarded (painted) surface bleeding through?

 

[footprints]

Could be old staining bleeding through try treating the area with two coats of a stain blocker loads in the DIY stores I use Zinsser Bulls eye 123 mostly but it is quite expensive min size tends to be 1 litre and quite pricey.
You can get stain blockers in a spray can which is fine for small areas.
Don't panic if the stain shows after the first coat, it will then be "locked in" and the second cover.

 

[dontbelieveawordofit]

looks like there’s water present.
there’s a crack where the cove meets the
ceiling. which we can assume you painted .
where’s the bath?