Evaluation of a method to determine discoloration of paints on wood due to tannin staining from knots

The natural exposure and water absorption tests carried out within European Standard EN927 use clear, knot-free wood in order to minimise variability in the results. However, knots are common in practice in many coated timber components, and it is widely acknowledged that the performance of paints can be compromised over knots or timbers that possess a high tannin content. As part of the European Standard for evaluating the performance of coatings on wood, a validated method is required for evaluating the discoloration due to tannin staining and resin exudation from knots. CEN/TC 139 Working Group 2 has been charged with identifying a reliable method of testing for tannin stain resistance and producing a standard; identified; identified as EN927-7 ‘Resistance to tannin staining’. This paper presents results from two laboratories on an evaluation of a Trätek standard procedure. Results of discoloration are presented for panels containing knots that have been coated with one of two test coatings. Reflectance spectrometry and digital imaging-based methods of evaluating discoloration were used and an interpretation of the comparability of the techniques is provided. The suitability of the method for promoting to WG2 as a standard is presented on the basis of these results.