Discolouration: Whilst poor quality paints frequently fade or discolour, especially when subjected to strong sunlight on exterior surfaces, discolouration can be caused due to the presence of harmful contamination within the surface or through outside agents within the atmosphere reacting with certain constituents in the paint. In a sulphur laden atmosphere, such as are present in industrial areas, a darkening of lead-based paints may occur due to the formation of black lead sulphide from the chemical reaction between sulphur compounds in the air and the lead pigments in the paint. Gradual discolouration can also occur due to the natural aging of the paint film which causes a gradual darkening of the paint medium. Remedy: Once paint has deteriorated by fading or darkening there are usually no effective means of restoring it other than by repainting, after taking any necessary practical steps to overcome the cause of the discolouration. It should be remembered, however, that poor quality paints are far more liable to suffer from this defect.