Evaluating the water regulating properties of finishes on exposed wood panels: Fid-Bau Portal

Evaluating the water regulating properties of finishes on exposed wood panels

Rypstra, T. / Eloff, A. / Kamden, P.

The primary task of exterior wood surface coatings is to maintain the in- and outward movement of water at acceptable wood moisture content levels. From quality coating systems it is further expected to continue to do so for a satisfactory length of time. Changes in the rates of movement of liquid water and water vapour through coatings on wood (or wood-based products) during service should, therefore, provide a good indication of the quality of coatings. The aim of this investigation was to develop a simple but effective method and to provide equipment to measure the water vapour permeability (WVP) and liquid water permeability (LWP) of coatings on wood panels; if possible at various stages of weathering. The development and evaluation of a prototype of the equipment to measure these properties on the exposed surfaces of outdoor panels, with dimensions 300mm x 100mm x 15mm, is reported. Mass increase of panels when a unit area is subjected to wetting or water vapour was used to derive the LWP and WVP coating properties. In the development of the method, the effects of shape and size of surface area tested as well as duration of the test were investigated. Water absorption rates were determined through four different rectangular unit areas of the exposed faces of test panels. These unit areas, measuring 50, 100, 150 and 200 cm², had the same 3:1 length to width ratio as the exposed faces of the test panels. It was established that the absorption rate measurements through a rectangular unit area of 200 cm² were more accurate, precise and sensitive. Further work is required to improve the equipment design and to apply the method to a wide range of surface coating systems at various stages of weathering and on different wood species and wood-based substrates.