There are many possible causes of the discoloration of paint films. It is often the effect of atmospheric pollutants on ingredients in the paint.
Exclusion from natural daylight may cause yellowing, of paints containing drying oils whilst exposure to bright sunlight may result in fading of some pigments. Some types of moulds or fungi can also cause discoloration of paints. When discoloration has occurred, there is usually no alternative to repainting, but if a recurrence of the defect is to be avoided, it is necessary to establish its cause and, if possible, to use materials resistant to the conditions.
If paint remains soft, tacky or even wet for a long time, possible causes are application in unsuitable conditions, e.g. poor ventilation, low temperature, excessive humidity, chemical pollution or application to surfaces on which there is grease, oil, wax polish or similar contaminant.
Drying may also be retarded if a preceding coat has not been allowed to harden sufficiently. The use of unsuitable thinners may also retard drying.
An improvement in atmospheric conditions, when these are the cause of slow drying, may allow the coating to dry eventually although it is likely that its appearance may be impaired and a further coat may be required.
When surface contamination is responsible, it will usually be necessary to remove the affected material, clean the surface thoroughly and repaint.
New plaster, rendering, brickwork and similar materials may contain soluble salts which, as the substrate dries out, are brought to the surface where they crystallise as a thin, hard film or a profuse, fluffy growth.
Efflorescence may also occur on aged surfaces if they again become wet, e.g. as a result of leaks or overflows. The fluffy type of efflorescence may disrupt paint coatings, particularly the relatively impermeable oil based types, if they are applied before the substrate has dried out and the growth has ceased. Emulsion paints may permit the salts to pass through the film with relatively little physical damage but they may affect its colour or appearance. Efflorescence should be removed with a dry, coarse cloth at intervals of 7 - 14 days and painting should be deferred until the growth ceases.
When efflorescence has disrupted a paint film the affected area should be stripped and repainting delayed until it is clear and the efflorescence has stopped.