LOSPs are used to protect timber against insects, including termites, and decay, though they are not intended for use in ground-contact situations. In addition, LOSPs can provide a degree of weather protection when water repellents have been incorporated. LOSPs are preservatives that leave the treated timber dry after treatment, and do not cause it to swell and distort in any way.
LOSPs have traditionally been colourless, and although it is now possible to obtain green and brown-coloured LOSPs, such colour that is imparted to the timber by these coloured variants is short-term. Also, it should be noted that some of the dyes used in coloured LOSPs might migrate through paint films that are later applied to the surface of the LOSP-treated timber.
The colour or general visual appearance of a piece of treated timber is not a reliable guide for identifying the actual preservative used. Users should check the marking (or branding) on the treated timber, or ask the supplier if in doubt. For example, some LOSP is tinted green, but this should not be confused with the green copper chrome arsenic (CCA) used in ground contact applications, for which LOSPs are unsuitable.
LOSPs and other preservatives like CCA are intended to provide adequate protection to a wide range of commodities but they may not necessarily offer directly equivalent performance over the broad range of applications. Most types of preservative-treated timber are subject to the effects of weathering when used in exposed outdoor situations. Any timber product used in such a situation will benefit from the application of surface coatings that provide additional protection against moulds, discolouration, checking and dimensional movement. LOSP-treated timber is generally more prone to these affects than CCA-treated timber, and it should always be given a protective coating after treatment.
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