Effects of Wood Roughness, Light Pigments, and Water Repellent on the Color Stability of Painted Spruce Subjected to Natural and Accelerated Weathering

This study examined the color stability of painted Norway spruce (Picea abies) samples subjected to natural and accelerated weathering, using Duncan’s tests and correlation analyses. The following effects were studied: (1) the different initial roughness of the wood; (2) use of transparent or lightly-pigmented top-coat layers; and (3) the presence of the final water-repellent layer. Natural weathering at a 45° slope in an industrial zone lasted 104 weeks, whereas accelerated weathering in Xenotest with 0.55 W/m UV irradiation at 340 nm and sprayed water lasted 12 weeks. The color stability of painted spruce, measured in a CIE-Lab system, was not, in the majority of cases, significantly affected by the initial roughness of the wood, the type of top-coat (WoodCare UV or PerlColor) layer, or presence of the final water repellent (AquaStop) layer. The light pine or larch pigments in the topcoat layers had positive color stabilizing effects. In their presence, the darkening (-L) and total color differences (E) of the painted samples dropped ca. 2.5 times during exterior weathering and ca. 5 times during Xenotest weathering. Samples painted with transparent coatings turned a reddish shade (+a) during the Xenotest, while those exposed to the exterior absorbed dirt and became more blue (-b).