Problem Solver - Distemper
Form of whitewash normally made using chalk or lime and a size in extreme cases lead was also used . You can identify by wiping your hand over the surface, is a white powder is present it is likely to be distemper.
Stage 1 - Preparation
Wash the surface with clean water to remove surface powder.
If mildew is present use a fungicidal wash to manufacturers recommendations.
Stage 2 - Basecoat
Once the surface is clean and all loose material has been removed, treat with an Alkali Resisting Primer or suitable plaster primer and allow to dry.
Stage 3 - Topcoat
Apply a suitable topcoat.