A modified laboratory method for assessing preservative barrier efficacy in refractory hardwoods: application to Eucalyptus nitens

Shallow penetration of preservative treatments in hardwoods like Eucalyptus nitens limits their suitability for above-ground outdoor use under Australian Standards. This study introduces a novel laboratory methodology to rapidly evaluate the performance of shallow CCA-treated barriers against fungal decay. Wafer-cube assemblies were constructed from untreated E. nitens boards and CCA-treated thin wafers, and exposed to accelerated decay tests using the brown rot isolate Fomitopsis ostreiformis. Various surface coating systems were applied to restrict fungal ingress to the treated face alone. Decay resistance was assessed using a visual rating scale from 0 (no decay) to 3 (severe decay), and supported by culturing assays to detect internal fungal penetration. Results demonstrated that coating system and barrier depth strongly influenced fungal exclusion, with latex + varnish and waterproofing + varnish systems achieving the lowest mean decay ratings (0.3–0.7) at 6 mm depth. These differences were statistically significant (χ² = 93.7, df = 30, p < 0.001). Fungal isolation frequency also declined significantly with increasing preservative depth and was strongly affected by coating type (p < 0.001, three-way ANOVA). The method enabled controlled comparison of barrier systems and demonstrated its utility as a screening tool for preservative performance in refractory species.<p></p>