The term knot yellowing, also known as knot bleeding, describes the spotty, yellow-brownish discoloration
of coated softwood surfaces. Scots pine wood (Pinus sylvestris L.) is particularly susceptible. Knot
yellowing is ascribed to the migration of wood extractives through the coating in combination with a
subsequent chemical reaction in the presence of light and oxygen at the surface and often occurs shortly
after painting (Black and Mraz 1974; Hse and Kuo 1988; Yazaki et al. 1985). It occurs on solvent-based
coatings in an alleviated way but particularely causes problems on water-based coatings, which prevail on
today’s European wood marked (Baa 2002). Despite substantial research and development work, the role
of specific extractives is still not fully understood. The objective of the present study was to characterize
the influence of stilbenes, lignans, resin acids and terpenes using pinosylvin, pinoresinol, abietic acid and
alpha pinene as model compounds.
Keywords: Knot yellowing, wood extractives, coating, UV light, pinus sylvestris
Authors
Ulrich Hundhausen
Julian Peitzmeier
Ingvild Johnsen
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