Bloom is a foggy, whitish (or blue-white), dull surface, which forms on varnished paintings. It is caused by moisture in the ingredients of the vanish or by the condensation of moisture on the surface of the image during its drying. The molecular action of the wet-film increases it temperature and the minute amounts of moisture condense of the surface to which it is applied.

There is no "best" varnish for the reduction of the chances of bloom. Every varnish will be dictated by the environmental circumstances when it being applied. There are, however, some varnishes which have a greater tendency of causing bloom than others. Damar, a commonly used varnish, is known to cause bloom. It is recommended as a glazing medium, as apposed to a final varnish.

Bloom is not a serious or permanent defect when it occurs on a well-varnished painting. It may be removed by placing a few drops of light machine oil (such as is sold for household use), on a flat folded tissue and rubbing the picture's surface with small circular and back and forth strokes, using as firm a pressure as the canvas can take without injury. The oil should remove the bloom immediately. Make sure there is no excess oil left on the surface. There are other methods for the removal of bloom but this is the most effective treatment.

Occasionally a painting will display a more serious type of persistent bloom, especially on older works of art. These are most likely internal and an expert in the restoration field would be highly recommended.