The surface characteristics of a painted product greatly influence the first impression. For many products, consistent color and uniform gloss are considered to be decorative quality criteria, which play an important role in the decision to purchase. Users should be aware that both effects are overlapping each other and influence the visual perception.
Changes in color and gloss are determined by regularly comparing the weathered samples to the original standard. The total color difference, dE*, is usually evaluated. Acceptable changes depend greatly on the hue. Brilliant colors tolerate greater deviations than dark and achromatic colors. In order to obtain additional information about the yellowing of the hue, often the db* is documented. The b* value represents the yellow/blue amount — the greater the deviation on the b* value, the more the paint yellowed. In case of light colors, the dirt pick-up is also rated. For this purpose, the change in lightness (L* value) is recorded. Figure 5 shows the results of an extreme accelerated weathering test with an alkyd resin. It is obvious that the 60° gloss value rapidly decreases and the color deviations dE*, dL*, and db* greatly increase.