Modern paints have excellent adhesion and covering power, but to get the best from them you must prepare the surface properly. To prepare previously painted woodwork, in good condition, use a fine grit sandpaper to remove any nibs and key the surface for repainting. For woodwork that may be in worse condition use a heavier grit sandpaper to remove flaking areas, then a fine grit to feather off the edges before repainting.
Previously painted ceilings and walls are best prepared by washing with Sugar Soap or a strong household detergent. Rinse thoroughly with clean water and allow to dry.
Remove areas of flaking, bubbled or blistered paint with a scraper and sand paper. Be sure to feather off the edges or fill the edges, with polyfilla or similiar. Sand then seal the bare plaster areas before repainting.
Cracks in the surfaces should be opened up with the edge of a scraper in a v shape. This is to ensure your plastering repair will be effective . Sometimes you may need to double fill as the first layer may shrink. Then you will need to get the sandpaper again! Sand it smooth and seal prior to first coat.