Light organic solvent preservatives (LOSPs) are regarded mainly as fungicides, with major toxicants of copper, zinc, tin or pentachlorophenol (PCP). Copper and zinc are normally used in napthenate formulations while tin is used as tri-n-butyl tin oxide (TBTO). These are used with a solvent such as white spirit. An insecticide can be incorporated, such as a synthetic pyrethroid.
Most LOSPs also contain waxes and resins that impart water repellence to timber surfaces. These reduce water uptake by the timber, thus minimising movement in service and decreasing the potential for decay. LOSPs are not suitable for use in ground contact. They are usually most effective where timber is to be given subsequent protection by painting or staining.
LOSPs do not swell timber whereas water-borne preservatives tend to raise the grain of timber and cause swelling. LOSPs are therefore suitable for the treatment of joinery, cladding and finishing products in their final form.
LOSPs are commercially applied by pressure impregnation of timber products. Other solutions suitable for on-going surface maintenance (water repellent preservatives) are readily available and can be applied by dipping, brushing or spraying.
Note: Safety precautions should be taken when working with LOSPs. In particular, PCP is thought to be a carcinogen. Older PCP preparations have been found to be contaminated with dioxins and furans, which are highly toxic.
Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.