Paint protects exterior wood surfaces from the elements.
Without a good coat of paint, exterior wood surfaces are at the mercy of wind, rain, sun, mildew and insects; but when they're protected by a layer of the right kind of paint, they can resist the damaging effects of all these forces. Some paints are better than others for exterior wood, and using the best option this year will prevent you from having to do it all over again next year.
Primers
Applying a coat of primer beneath the paint on exterior wood surfaces is a crucial step in ensuring that the paint job will last. Primers seal the wood and provide a consistent, smooth surface to which the paint can bind easily. The primer also acts as a moisture barrier, preventing the saturation of the wood by exterior moisture, which can lead to decay. In addition, primers keep moisture and chemicals from seeping out of the wood and through the paint, which would result in peeling and staining of the paint coat. Primers may be acrylic-, oil- or alkyd-based.
Acrylic Paints
Acrylic paints consist of pigments suspended in a synthetic polymer medium. When acrylic paints dry, they harden into tough, flexible coatings that adhere well to surfaces. They are water-soluble, which means that cleanup of brushes and tools is easier and less messy than with oil- and solvent-based paints. Because of its flexibility, acrylic paints are well-suited to exterior surfaces, where temperature and humidity changes cause materials to expand and contract.
Alkyd Paints
Alkyd paints have largely replaced natural oil-based paints, which were typically made using a linseed oil medium. These newer paints are based on a synthetic medium to which fatty compounds have been added. Alkyd paint cleanup requires mineral spirits or other solvents. Because alkyd coatings are relatively inflexible, they are not a good choice for exterior wood. Avoid painting over surfaces that have already been painted with an alkyd paint with an acrylic paint; the acrylic will not adhere well to the alkyd surface.
Polyurethane Paints
Polyurethane paints are oil-based paints designed specifically to provide a durable surface that will hold up to heavy wear. They resist oil and dirt, are washable and will not scratch or abrade easily. They are appropriate for use on exterior wood surfaces such as porches, decks, stairs and railings. Surfaces must be thoroughly dry and free of dirt, oil, mildew, flaking paint and other contaminants before you apply the polyurethane paint.
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