Semitransparent stains for wood products are favored for outdoor applications in Japan because these finishes do not hide the wood grain. However, the photoprotective effects of wood surfaces are low, therefor there is a need to improve the weathering performance. We examined the natural weathering performance of semitransparent stains on wood pretreated with a copper monoethanolamine solution (CuA) to improve its photostability. Untreated and pretreated with CuA Japanese larch (Larix kaempferi), Todo fir (Abies sachalinensis), and Japanese cedar (Cryptomeria japonica) wood panels were finished with semitransparent stains containing a different type of resins and colours and exposed outdoors in Hokkaido, Japan for 24 months. Results from the visual assessment and statistical analysis using the Kaplan-Meier estimates of survival analysis showed that the formation of cracks predominantly observed and the stains performed better on pretreated than untreated panels. We conclude that the natural weathering performance of semitransparent stains can be enhanced by using CuA pretreatment. However, further studies would be needed to clarify the reason why the effectiveness of CuA pretreatment differ from resin type and/or wood species.

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