Long-term appraisal of on-site preservative treatments for preventing decay in exterior woodwork

This research was initiated in 1958 to investigate the efficacy of various preservatives and treating methods for new lumber intended for exterior structures. Post-rail units (2 by 4 in. (5.1 cm by 12.7 cm) constructed of Southern Pine sapwood, Douglas-fir heartwood, and mill-run western hemlock were dip- or brush-treated before or after assembly. Units were treated with pentachlorophenol (penta) in various petroleum solvents or with copper naphthenate in mineral spirits. Both painted and unpainted units were exposed on a test fence in Madison, Wisconsin. Most of the painted untreated pine units (controls) failed by 34 years after treatment. Surprisingly, painting completely protected untreated Douglas-fir heartwood units from decay and afforded substantial protection to untreated hemlock. For painted treated units, significant decay was present only in lightly treated pine units (2-s dip after assembly or brush treated). Copper naphthenate (1% Cu) was markedly less effective than penta treatments on unpainted pine and hemlock units. There was no evidence that type of oil carrier or incorporation of a water repellent improved effectiveness of treatment. Three-minute and 15-minute dips in penta were equally effective. Application of penta-grease to only the ends of units was effective.