Painting of wood surfaces exposed to sunlight (UV) can easily lead to paint defects and failures. Typically wood windows, doors, purlins, eaves, siding, pickets, planks, lumber, timber, gluelam, posts, frames and railings are subject to strong UV oxidation and degradation. “Green” wood is considered to have a moisture content (MC) of ~40% and should not be painted. Kiln or oven dried wood can be painted provided the moisture content has decreased to 12% - 15%, since at these levels the MC in many wood species has settled to an equilibrium value. Using a Delmhost moisture meter I have measured dry wood with moisture below 6%. When exposed to sustained high humidity, unpainted dry wood can absorb moisture to levels >15%, and these surfaces should not be painted. Water in the form of rain, or leaks from buildings can also penetrate wood, unacceptably raising the moisture level. All excessive moisture must evaporate out of the wood before it is painted, since the painting of moist wood can lead to blisters, paint peeling and flaking due to loss of adhesion. Blistering is caused when evaporating moisture pushes the primer and topcoat from the wood surface. Water often gets into wood through the end grain, and then migrates inward along the grain lines. To prevent this, the painting contractor must properly seal all end grain with a suitable exterior wood sealer followed at least with a primer and top coat. Manufactured wood products, such as windows and doors are usually pre-primed by the manufacturer using a high quality shop primer. It is critical that the on-site painting contractor apply the primer and topcoat to windows, doors, etc., within days after the products arrive on site and are exposed to UV light. Photo-oxidation of the wood surface fibers takes place soon after being exposed to sunlight, and paint adhesion can easily be compromised. Many adhesion failures are caused when the contractor waits too long before applying the wood primer and topcoat. In some cases even two weeks is excessive. Most product components, such as wood window frames, muntins (mullions), rails, stiles, sills, wood door frames, panels, etc. should be painted on all six sides to prevent ingress of humidity and rain. Finger joints and miter joints must at the very least be properly sealed. Gaps between wood windows, doors, other wood construction members etc. and adjoining concrete and stucco walls must be caulked to prevent moisture penetration. The architect, builder, general contractor, or painting contractor (whoever is responsible) must select a suitable high quality primer and topcoat to be applied. An oil based alkyd primer is popular, and this is followed by one or two coats of 100% acrylic latex topcoat. In some instances water based (waterborne) acrylic latex primers are specified. For wood species that have high sap contents, it is recommended that a stain blocking primer (stain blocker) be applied to prevent staining and bleeding. High quality sealers, primers and top coats should always be used since they are designed to resist chalking, fading, loss of gloss, cracking, crazing and prevention of mold and mildew growth. High quality topcoats contain mildewcides (mildicides) or biocides. UV light absorbers in high quality paints prevent them from becoming brittle, loss of gloss, cracking, crazing, splitting, chalking etc. In addition, these paints also contain high quality titanium dioxide (TiO2) and other pigments or extenders, such as aluminum oxide (alumina), silica, etc. that protect the paint from degrading in sunlight. Typical exterior topcoats are made from 100% acrylic latex, acrylics modified with siloxanes, polyurethanes, and more recently PVDF. For clear finishes, marine varnishes, polyurethanes, 100% acrylics and PVDF are used. Before selecting a coating system for exterior wood surfaces, you are always advised to consult your local paint store, (not the typical salesperson in a hardware store). Most reputable paint manufacturers publish detailed instructions on how to select and apply their paints. |
