Water spotting is the visual defect caused by the water on the surface to be painted or on the paint film, on the surface appearance of the paint film. The defect may appear in the form of misty spots, rings or dull spots.

Spotting is usually caused by the paint's sensitivity to water. In addition, a defect may occur when substances dissolved in water precipitate on the paint film as the water evaporates. While water spots are easier to observe on glossy topcoats, the effect can also be caused by the precipitation of various impurities on the paint applied surface or on the underlying primer coat. Sensitivity to water can be caused by insufficient curing, presence of hydrophilic solvents, amines, other polar substances in the film, etc. Salts, acids or other substances accumulated on or dissolved from the film may affect the curing of the paint, creating curing problems or causing wetting problems of the subsequent coats.

Fig 1.

In order to take action against water spotting correct diagnosis of the problem is necessary. If the reason is that hydrophilic solvents remain in the film, these are replaced with hydrophobic solvents. If the paint is insufficiently heated, the heating time is extended, the heating temperature is increased, etc. Sometimes a hydrophobic binder composition may be necessary. Washing and/or rinsing after processes such as metal surface preparation, casting of the plastic part and sandblasting should be reviewed for the quality of the rinsing water and the inadequacy of washing and rinsing. Surfactants may need to be added to make the rinse water wet the entire surface instead of forming water droplets that will leave stains when dried.