Cedar is a popular material for shingles and siding because natural oils in its heartwood (the dark red wood at the center of the tree) make it resistant to the weather and hungry insects. Even unpainted, cedar will last for years. Painting cedar, however, calls for some preparation.
Some sections of old, unpainted shingles and siding may look darker than others. This discoloration is most likely caused by a combination of excessive tannins leeching from the wood and mildew growth. You'll have to remove the mildew and prime the wood with a stain-blocking primer. You can prime and paint new shingles as soon as you have installed them, but since weathering can quickly alter the paintability of wood, if you don't get to the job within two weeks, you'll have to recondition the wood fibers. Powerwashing old shingles is not recommended. Weathering makes them soft, and it's almost impossible to avoid gouging them and to properly dry the water forced under them.
Survey the entire area of your house, especially the lower levels, where the shingles will more likely retain water. Wherever you find mildew, sponge on a 1-to-3 bleach-water mixture, scrub it, and let it soak in for 20 minutes, keeping it wet during this period.
Using a garden hose with a moderate spray, rinse off the bleaching solution thoroughly. Let the area dry completely, which can take as much as two weeks.
Recondition the entire surface to remove aged and weathered fibers in the wood. Use a stiff scrub brush, working the bristles into the overlapped edges along the bottom of each course and drawing the brush down the face of the shingle. Brush off the dust with an old 4-inch paintbrush.
Using a 4-inch brush, apply an oil-base stain-blocking primer. Work the brush into the overlapped edges and the recesses between the shingles. Let the primer dry and apply a high-quality acrylic latex house paint. If you roll either primer or paint, back brush so all surfaces are protected.
Within two weeks of their installation, prime the shingles with a stain-blocking primer. (If you wait longer than that, recondition the surface as shown in step 3 above.) Apply the primer with a 4-inch brush, starting along the overlap and painting a length of about 4 feet.
Once you have primed a section of the overlap, paint the face of the row, working the paint in all directions and between the shingles. Smooth the application with downward vertical brush strokes and repeat the process on the next section, always working toward a wet edge.
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