MOVAP: performance of exterior wood finishes
Background to the study
In Europe, the performance criteria for finishing systems are listed in EN 927-2. They are based on the mandatory assessment of blistering, cracking, peeling and adhesion after 12 months of natural ageing (EN 927-3), as well as on the assessment of water absorption (EN 927-5). For spray-applied coatings, the presence of micro-bubbles in the dry film should also be assessed if the finishing system is intended for the “stable” use category (as defined in EN 927-1). Blistering is a sign of a lack of water vapour transmission through the coating film. It is rarely encountered with current waterborne finishes.
Therefore, of the performance criteria listed above, the first visible sign of coating degradation during ageing and the life of the structure is cracking. This cracking of the finish then leads to flaking of the coating and loss of adhesion. Once the film has cracked, the water barrier provided by the finish to the wood material is no longer guaranteed. Anticipating this cracking, and above all avoiding it, is a major challenge for the development of high-performance finishes for exterior wood.
During the MOVAP project, the artificial ageing of two finishes was studied by analysing variations in Persoz hardness and gloss, in addition to the traditional visual ratings recommended by the EN 927 standards.
Two finishes were applied in 3 coats on Scots pine, according to the recommendations of their technical data sheet. Finish 1 was a white water-based acrylic resin paint. Finish 2 was a semi-transparent solvent-based alkyd resin glaze.
Visual changes produced by ageing
The white paint (Finish 1) showed no defects during ageing and no cracking was observed. The semi-transparent stain (Finish 2) showed a more marked change in appearance which the operator’s eye attributed to changes in gloss during ageing. It did not show any visually noticeable cracking. Both products are therefore considered to perform well.