The goal of a durable exterior clear coating has eluded generations of coatings technologists, despite long-standing desire amongst the public for such a coating. The journey towards this goal initially focused on modifications to coating formulation, but took a completely different direction when it was found that a UV-transparent silicone clear coating on wood modified with chromic acid met consumer expectations of coating durability. This finding sparked world-wide interest in wood pre-treatments as a way of enhancing the durability of clear coatings. This interest initially focused on transition metal compounds, but has now shifted in the direction of organic and inorganic photostabilizers or even more drastic pre-treatments. Pre-treatments that dimensionally stabilize wood, protect it from microbial degradation and photostabilize lignin, when combined with flexible, photostable, coatings provide the next way-stop on the journey towards achieving the goal of durable exterior clear coatings for wood. This paper reviews this journey, drawing upon our research and that of other groups who have focused on this elusive goal.

Chemical modification of the molecular structure of polymers is a fundamental approach to improving the resistance of materials to photodegradation, but it is seldom employed because other techniques, such as the use of additives (ultraviolet absorbers and antioxidants) can provide adequate levels of protection. Wood is an exception to this rule because the additives that can protect polymers from photodegradation are less effective at photostabilizing wood. Conversely, chemical modification of wood surfaces with chromic acid is remarkably effective at photostabilizing wood, and this led to significant interest in chemical modification as a way of protecting wood from photodegradation. Early research focused on understanding why chemical modification with chromic acid was so effective at photostabilizing wood, and significant effort was directed towards finding effective alternatives to chromic acid. This search was largely unsuccessful and hence attention then shifted to the development of alternative methods of chemically modifying wood to improve its photostability. Research was also performed to examine the extent to which chemical and other modification systems designed to improve dimensional stability and decay resistance could protect wood from weathering. This paper reviews this research and describes the progress made to date to develop modification systems that can block the photodegradation of wood.

Remarkable increases in the performance of complex systems can be achieved by a collective approach to optimizing individual factors that influence performance. This approach, termed the aggregation of marginal gains, is tested here as a means of improving the performance of exterior clear-coatings. We focused on five factors that influence clear-coating performance: dimensional stability of wood; photostability of the wood surface; moisture ingress via end-grain; coating flexibility and photostability; and finally coating thickness. We performed preliminary research to select effective wood pre-treatments and durable clear-coatings, and then tested coating systems with good solutions to each of the aforementioned issues (factors). Red oak and radiata pine panels were modified with PF-resin, end-sealed, and thick acrylic, alkyd or spar varnishes were applied to the panels. Panels were exposed to the weather and the level of coating defects was assessed every year over a 4-year period. All of the coatings are performing well on PF-modified pine after 4 years’ outdoor exposure. In contrast, coatings failed after 2 years on unmodified pine and they are failing on PF-modified oak. We conclude that our approach shows promise. Future research will build on the current work by developing solutions to additional factors that influence clear-coating performance.