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.