Sugi heartwood specimens finished with one of 6 water-borne (WB) or 2 solvent-borne (SB) commercially available wood preservative semi-transparent coatings were exposed to the weather at 45° inclination facing south in Tsukuba for up to 24 months, and changes in color, surface defects and water repellency of finished surfaces were monitored. The level of coating opacity was also assessed by measuring contrast between early- and latewood on unexposed specimens to examine its relationship to coating performance. The weatherability of WB coatings varied widely. After 24 months of exposure, color differences (ΔE ^*ab) for specimens finished with WB and SB coatings ranged 10-50 and 10-20, respectively. WB coatings that showed smaller color differences also had relatively few defects as were the cases for SB coatings. Water repellency was maintained above 80% for SB coatings after 18 months, while it decreased more quickly for most of WB coatings. There was a tendency that color changes and surface defects decreased with increasing level of hiding grain, regardless of WB or SB coatings. This indicates that color changes and surface defects may depend on the transmission of light through coating layer. No similar tendency, however, was observed for water repellency, suggesting that different mechanisms may be involved for the performance of water repellency.

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