Discolouring fungi reduce the service life of coated wooden claddings in façades and increase the total cost of ownership due to shorter maintenance intervals. It is of major importance to find paint systems for wood which are durable and have a high resistance to mould growth. A total of 15 paint systems from 9 different manufacturers were exposed on two field sites, Sørkedalen and Birkenes, in Norway. Finishes included in the study were water-borne and solvent-borne alkyd primers, water-borne mixed alkyd/acrylic paints, water-borne alkyd paints, water-borne acrylic paints and solvent-borne alkyd paints in different combinations. Untreated spruce was used as a control. The panels were exposed outdoors for 16 months and the degree of mould growth was estimated by visual inspection every second month except during winter time. The outdoor test procedure was based on EN 927-3. Temperature, precipitation and relative humidity data were collected on each site. The paint systems had the same mould growth performance in both test sites after 16 months. Although average temperature and precipitation were higher in Birkenes, the mould infestation appeared faster in Sørkedalen which had a higher average RH. Pure water-borne systems performed best in terms of the degree of mould growth and systems with a solvent-borne alkyd top coat performed poorest. The paint systems with lowest degree of mould growth had IPBC as the only fungicide in the top coat and a mixture of several fungicides in the primer. With few exceptions, paint systems with an alkyd top coat, either solvent-borne or water-borne, have higher degree of mould growth than acrylic and mixed alkyd/acrylic top coats.

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