Search

Film defects in waterborne coatings for exterior joinery - Their formation and influence on performance

Svane, P. / Jensen, L. / Miller, R.

Film defects, mainly air inclusions in water borne coatings, have been studied by a literature review and a programme of laboratory experiments. The influence of defects on the exterior performance of coatings on wood has been evaluated in a natural exposure trial. The formation of air inclusions is complex, and depends on a number of factors including substrate, paint type, paint system and formulation, and application equipment and procedure. Effective control of these factors by manufacturers and users can reduce film defects to an acceptable level. This work had the objective of providing guidelines for achieving successful results. It has been shown, that air inclusions adversely affect the performance of coatings on exterior wood in a number of important respects. Of particular interest is the development of craters from eroding air inclusions which facilitate colonisation by blue stain fungi and thus contributing to coating brakdown. Air inclusions were not found to influence moisture content levels in exposed wood samples during the first year of exposure. Work is continuing to establish longer-term trends. Other factors shown clearly to influence moisture levels in coated wood include: Coating system, wood species and pretreatment. Among the coating systems, solvent borne alkyd paint produced lower panel moisture contents than water borne coatings. A solvent borne double vacuum treatment was similarly more effective in controlling moisture contents than its water-borne counterpart. Pine heartwood consistently exhibited markedly reduced moisture levels compared with pine sapwood.