Blistering can occur on plaster if you overcoat solvent-based paints such as gloss or eggshell with a conventional emulsion in an area that suffers from high levels of condensation. To resolve the problem, scrape back the blistered paint until you have a firm edge, feather lightly with abrasive paper and dust off. Now spot, prime and bring forward any bare areas. Finally, repaint the surface using a thinned first coat of water-based eggshell followed by one or two full coats.