Dark patches around plasterboard patch repairs after painting

[markjg123]

Prior to decorating, I removed an unwanted phone socket and patched the hole with a square of plasterboard before filling with a couple of mm Toupret finishing skim. Once dried I then painted the whole room using Dulux matt emulsion. It looks great apart from a dark border around the plasterboard patch. It's been 4 days since I painted. Any thoughts on the cause of the problem and ways to fix? I thought about using a stain block then another coat or two of emulsion.

 

[Wayners]

Re skim coat.

Surface was dry but not deeper. Maybe damp and you sealed it in with emulsion and now it's showing through.
Probably best to leave it a course of weeks then emulsion over.
If that don't work then stainblock and paint.

 

[markjg123]

OK I'll do that. If it was damp has the emulsion prevented it from drying out at all?

 

[davidm1969]

What was behind the phone socket that you removed? If it was a wire through the wall to the outside this could be bridging any cavity and/or causing a cold spot and the damp? Solve the problem if you can
Good luck

 

[markjg123]

What was behind the phone socket that you removed? If it was a wire through the wall to the outside this could be bridging any cavity and/or causing a cold spot and the damp? Solve the problem if you can
Good luck

Just a small cavity to what I think was breeze block. It is an inside wall. I live in a flat in a converted mill. I think all that was left were the external stone walls and slate roof. From what I've seen my guess is the internal structure has been constructed with breeze block then plasterboard has been overlayed on timber studs.