Performance Evaluation of Exterior Wood Stains Under Laboratory and Field Scale Test: Focus on Water Repellency, Color Retention, and Fungal Growth Resistance

Wood stains are developed to enhance the aesthetic appeal and durability of wood by improving water repellency (WR) and protecting surfaces from biological decay and UV degradation. However, limited information is available on whether these performance characteristics are adequate to ensure the long-term protection of commercially used stains. This study evaluated 55 commercial wood stains, categorized by solvent type (water- or oil-based) and pigment transparency (semi-transparent or opaque), focusing on WR, color retention, and fungal decay resistance under both laboratory and outdoor conditions. Initial WR correlated with reduced moisture uptake but was insufficient to prevent fungal growth and decay. This suggests that the presence and effectiveness of additives, such as fungicides, are necessary to enhance the decay resistance. In the field-scale test, the opaque water-based stain WBO1 exhibited the best performance in maintaining WR (>90%) and color retention (∆E < 8.0) at both test sites, and suppressing discoloration fungal growth, despite its relatively low decay resistance in the laboratory test. These findings indicate that the color retention of pigments, which effectively block UV radiation, is important for the long-term durability of wood stains. This study underscores the need for improved stain formulations that more accurately reflect long-term outdoor performance.