The ceiling is skimmed plasterboard. It has previously been painted with emulsion paint. Then it has had wallpaper pasted over it. Then it has had the wallpaper taken off, using a steam wallpaper stripper.
At present, the ceiling in general has over its surface a variable, thin film of wallpaper paste. Is it essential to remove by washing all the wallpaper paste before applying fresh emulsion paint, or will a slight film of wallpaper paste cause no problems for the emulsion paint?
Also, the steam wallpaper stripper has also removed irregular strips of the original emulsion paint, thus leaving very shallow depressions in the ceiling surface. The largest strip is around one metre long by five centimetres wide. What are good ways of dealing with this problem?