We've just had our kitchen ceiling replastered and now it's been that it's painted, there are a couple of areas that look like oily patches underneath the paint. The plasterer has told us to paint over the offending areas with undercoat, (which we have done) then repaint over with the white emulsion that we painted the ceiling with, but these marks have reappeared, we have now done this process twice to no avail, any suggestions as to how to sort this problem?