Application and artificial weathering performance of translucent coatings on resin-treated and dye-stained beech-wood

Beech wood boards (Fagus sylvatica L.) modified with thermosetting N-methylol melamine (NMM) and phenol-formaldehyde (PF), partly with a dye added (NMM-dye, PF-dye), were coated solely with a translucent water-borne acrylic binder and a formulation containing the same binder and UV-protective agents (UV-PA: UV absorbers, HALS). The aim was to improve the weathering performance and to maintain the colour stability of the modified wood. The applicability of the coating on the modified boards was similar to that of untreated control boards, but the drying times were prolonged. Capillary water uptake of modified, uncoated and coated boards was clearly lower than that of respective controls. Dry and wet adhesion of the coating film and the proportion of wood fracture were higher on the modified substrates than on the controls, but no clear difference was observed among the modified boards. The colour stability during weathering depended in the modification and coating. Untreated and NMM modified samples became lighter, while PF-modified boards turned darker. The weathered coating on the modified boards, particularly with PF resin, showed less blistering, flaking and cracking than that on the controls. The coating on all resin-modified boards maintained higher adhesion during weathering than the controls. UV-PA stabilised the colour and adhesion on all boards compared to the single binder formulation. The study shows that wood modification with NMM and PF improves the weathering performance of wood coated with acrylic coatings. Combined modification with staining can diversify the optical appearance of wood coated with translucent finishes.