Past water leaks or burst water pipes can cause water staining. Before repainting, ensure that the leak has stopped and the surface is thoroughly dry, then spot prime the affected area with a good primer. Heavy deposits of tar staining from cigarette smoke or soot staining from open fires are common causes of staining, especially on water-based emulsion paints. To prepare, wash the surface thoroughly with detergent solution and rinse frequently with clean water. Allow to dry, then prime. Conventional solvent-based finishes such as gloss can discolour with age, particularly in areas sheltered from natural daylight, this is known as yellowing. Prior to repainting, wash the surface with a detergent solution, rinse with clean water and allow to dry. It may be more advisable to use a water-based coating for a low light area.