As with interior painting, proper surface preparation is crucial for achieving a long lasting paint job. A thorough cleaning to remove dirt and mildew is the first step in preparing exterior surfaces.
All loose paint deposits and defective caulking should then be removed. Any rotted boards should be replaced, and all new and bare wood primed. Gaps between adjoining wood, or wood and masonry, should be caulked with a quality caulk or sealant. Hardware and light fixtures that are not to be painted should be removed or properly protected, and all plants should be moved or covered.
Rusty surfaces, such as steel lintels and wrought iron, may require special preparation. All loose rust should be removed, and the surface treated with a rust converter, primed, and then completed with the appropriate topcoats.
As with interior paints, many manufacturers have developed new acrylic products which offer in many cases superior performance over oil base paints.
A 100% acrylic paint should be used for exterior surfaces. Less expensive paints generally contain a vinyl, or vinyl-acrylic, resin, which is not as durable as a 100% acrylic resin and does not retain colour as well.
Generally, paints offered for residential use are either oil or water base (acrylic). The base indicates the solvent used to suspend pigments (colouring agents) and resins (binding agents). Because they dry slowly, oil paints should not be used for exterior painting in warm, humid weather. The slow drying time gives mold spores more opportunity to attack the drying paint film.
This, combined with the nature of the resins in oil paints, provides a near perfect environment for mildew. Acrylic paints are much more mildew resistant. While most high-quality paints now contain mildewcides, such additives will only inhibit mildew growth, not prevent its occurrence.
Oil paints also become very hard and brittle. When exposed to intense sunlight, particularly on surfaces facing south or west, the paint film expands. The hard surface eventually cracks, permitting moisture to get between the paint film and the substrate.