An Australian Test for Decay in Painted Timbers Exposed to the Weather

There are no Australasian data for the comparative performance of painted timbers exposed to the weather. In this test, seven untreated timber species and two impregnated with a proprietary copper-chrome-arsenic preservative salt (CCA) were exposed to the weather at 24 sites in Australia and 3 in Papua New Guinea. All of the test panels were painted prior to exposure. The natural durability (decay resistance in ground contact) of the untreated timbers ranged from very low to moderate. The first assessment for the presence and extent of decay was made after an exposure period of 3 years. At the conclusion of the examination, the test specimens were repainted and returned to their respective sites for further exposure. The test specimens were assessed for their resistance to decay aboveground by two methods. The order in which the two methods ranked the timber species and treatments for their resistance to decay aboveground was identical. The only untreated timber to be recovered completely free from decay was brush box (Tristania conferta). The heartwood of ramin (Gonystylus spp.) performed poorly, as did the sapwood of radiata pine (Pinus radiata). However, when impregnated with a CCA preservative and painted, the sapwood of radiata pine performed as well as the brush box and, at the time of this examination, was performing better than the heartwood of mountain ash (Eucalyptus regnans) treated to the same retention of the same preservative. The sites chosen for this test were ranked for the severity of their hazard from decay aboveground by two methods, and the results so obtained were compared with Scheffer’s climate index (6) for estimating the potential for decay aboveground. Statistical analysis of the data showed evidence of a relationship, but the association was not a strong one.

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