have just had a new velux installed and new ceiling i.e., plasterboard. Unfortunately the disturbed slating around the velux has leaked but now put right. The ceiling had been skimmed, a watered down coat of emulsion and two top coats of dulux white vinyl matt. The rainwater has dried but badly discoloured the new paintwork. The builder has told me to give it a coat of oil based undercoat and then emulsion again and said it will not bleed through again. Is this a good solution/good practice?