What about do-it yourselfers who want to work with cedar siding or shingles that have already been finished? The FPL report, Installation, care, and maintenance of wood shake and shingle siding, can help with that.
Finished surfaces (paints, semitransparent stains, and solid-color stains) often develop mold and algae. As with wood surfaces, these microorganisms feed off airborne contaminates, but can also metabolize oils in some finishes. Surfaces of paint and solid-color stain can easily be cleaned with commercial cleaners or a dilute solution of household bleach and as with wood, the gentler the better. Abrasive methods will damage the finish surface. It is more difficult to remove these microorganisms from saw-textured wood; the microorganisms may be inter-grown through and under the finish.
Wood Repellant Preservatives (WRPs) and Semitransparent Stains
Other than mold removal and gentle washing, WRP and semitransparent stains should not require additional surface preparation. They can be reapplied as soon as shakes or shingles are dry following cleaning. The reapplication of semitransparent stains and tinted WRPs requires special care. Usually residual finish is left in some areas, whereas the finish may be completely weathered away in other areas. The weathered and unweathered surfaces accept penetrating stains differently; the weathered areas absorb stain easily, whereas stain forms a second coat on areas having residual stain. During application, feather the stain into areas having old finish to avoid forming this second coat. If a second coat is formed, it will give a shiny appearance and will fail by flaking. When refinishing a structure, it is often necessary to have the north, east, west, and south sides on different cycles. Semitransparent stains usually last 4 to 5 years on the south side and more than 10 years on the north side. These estimates can vary considerably depending on the design of the structure, exposure conditions, and the quality of the shake or shingle. As with the application for new construction, take care to avoid lap marks. Changing colors or switching to a paint or solid-color stain is not an option with semitransparent stains unless the old finish and weathered surface is removed. The surface must be scuff-sanded, primed, and finished (at least two coats may be required to hide residual stain).