Factory-finished shakes and shingles are available and in recent years have become the preferred product in many areas of the country. Installation, Care, and Maintenance of Wood Shake and Shingle Siding outlines how factory finishing offers several advantages over on-site finishing.
Factory finishing eliminates weather limitations and avoids damage by UV radiation to the unpainted wood. Back priming (the application of finish to the back side of the shake) of shakes and shingles is easily done during factory application. Back priming decreases water absorption and extractives bleed. The primer should extend about half to two thirds up the back side of the shingle. If pre-finished shakes or shingles need to be trimmed during application, the cut surfaces should be touched up with finish.
It is possible to finish shakes and shingles prior to installation. It is a rather labor-intensive process, but will improve the performance of finishes on shake and shingle siding, particularly on structures having minimal roof overhang and on the sides of structures exposed to strong wind-blown rain. Finishing shakes or shingles prior to installation makes it possible to back-prime with a stain-blocking primer or penetrating stain.
The most effective means for applying finishes to wood is by brush. Brushes are an investment and it is wise to purchase top quality brushes, as they will last for a long time if properly cared for. For latex-based finish, use a synthetic bristle brush; for oil-based finishes, use a natural-bristle brush. For best results, purchase long-bristle brushes. If you choose to apply the finish by spraying or with a roller, brush the surface (back brush) immediately after application. Back brushing is essential to ensure that the finish is spread evenly and worked into the wood surface. Ensure that the butt-end gets a liberal application of finish; this is the most important surface. This is especially important for semitransparent stains and tinted WRPs. If using a roller, use natural roller covers for oil-based finishes and synthetic rollers for latex-based finishes.
For tinted WRPs and semitransparent stain application to installed shakes and shingles, it is necessary to take care to prevent lap marks (an area with two coats rather than one). Apply the finish in a single direction, usually across the structure, from a corner to a door, window, or other corner, keeping a wet edge throughout the application of this section. The lateral edges of the advancing strip of finish must coincide with the top and bottom edge of the course (s) of shake or shingle. When applying finish adjacent to the course that is already finished (the next strip), take care not to apply additional finish to the area that is already finished, as this causes a lap mark. For penetrating finishes such as WRPs, semitransparent stains, and to some extent solid color stains, lap marks give an unsightly blotchy appearance.