Natural weathering performance and the effect of light stabilizers in water-based coating formulations on resin-modified and dye-stained beech-wood

This study assesses the performance of untreated and resin-modified beech-wood (Fagus sylvatica L.) during outdoor weathering. Boards modified with thermosetting N-methylol melamine (NMM) and phenol–formaldehyde (PF) resins, which were partly dye stained, were coated solely with a waterborne acrylic binder and formulations containing the same binder with different types and contents of photo-protective additives. Most modifications of the wood substrate changed the original color of wood, except for sole NMM modification. Changes in mass and capillary water uptake during exposure of modified, uncoated and coated boards were less than those of respective controls. Surface defects and cracks were clearly fewer on modified wood than on the controls, but no clear difference was observed among the topcoats containing UV-protective agents (UV-PA). The color stability during outside weathering depended on the treatment and coating formulation. Untreated and NMM-modified boards became grayer, and the NMM-dye-modified boards turned to a lighter gray, while PF-modified boards adopted a darker, blackish color. The weathered coating on the modified boards, particularly with PF resin, showed less blistering, flaking, and cracking than that on the controls. UV-PA stabilized the color and adhesion on all boards compared to the sole binder formulation. We conclude that wood modification with NMM and PF resin improves the natural weathering performance of wood coated with acrylic coatings. Combination of modification with staining enables diversification of the optical appearance.