Within two months of exterior exposure to sunlight, all woods will begin to turn yellow or brownish, then gray. Dark-colored woods eventually become lighter and light-colored woods become darker. Surface checks and cracks may develop, the grain raises and loosens, boards cup and warp pulling fasteners loose and the wood becomes friable with fragments separating from the surface. After the initial weathered surface has developed, usually within 1 to 2 years, further changes are slow to develop.
Where painting is being considered, wood surfaces roughened from weathering provide a very poor substrate for any film-forming coating. Even a few days of exposure for a new, untreated, clean wood surface will decrease it's paintability and the overall life of the paint. But slightly weathered surfaces may be beneficial for penetrating finishes because they allow the wood to absorb more of the finish solution. If wood species or conditions are less than the most desirable for painting must be used, a non-film-forming finish, such as a water repellent or a semi-transparent penetrating stain should be considered instead of paint.
Exterior plywood will develop surface checks with moisture and sunlight. Quality acrylic latex stain-blocking primer (80230) and acrylic topcoat (20 series or 21 series) systems perform best to help avoid this problem.
Leafing aluminum (280209) as a primer, topcoated with a quality exterior latex coating (9 series or 7 series) has also shown excellent results on plywood. Plywood should never be left unfinished with exterior exposure. The natural weathering process degrades the thin surface veneer quickly. Transparent finishes are also unsuitable because they do not protect the surface from weathering.
Most "treated" lumber available at lumber yards is pressure treated with a waterborne salt to form an insoluble residue which helps offer protection against ultraviolet degradation. The greenish brown color has a clean, paintable surface as long as the pitch or extractives are not "bleeding" through to the surface. Pitch can be scraped off and wiped with clean paint thinner. After drying, a quality acrylic primer (80230 or 7-225) and a quality breathable acrylic topcoat (20 series or 21 series) can be easily applied with good results. It is generally wise to age new "treated" deck lumber approximately 1-2 months to allow pitch and volatiles to exude prior to painting.