Good surface preparation practices are essential to achieve a durable, long-lasting exterior finish. Most premature exterior coating failures are attributable to inadequate surface preparation resulting in:
- poor finish coat adhesion
- surface discoloration
- lack of finish coat uniformity (sheen, color and surface hiding)
- lack of corrosion resistance
The first step in surface preparation is to inspect the surface and make any necessary repairs. The surface must then be cleaned to be free of all dirt, mildew and loose material (discussed below).
Paint and Solid Color Stains
Exterior Western Red Cedar surfaces only need to be refinished when the old coating has worn thin and no longer affords the wood protection. In refinishing painted or solid-color stained siding and trim, removal of the old coating may be required. This is necessary if, for example, the old finish is severely cracked or is peeling. These finishes can be removed by a variety of procedures, all of which can be difficult, time-consuming and expensive processes. Some of these procedures can damage the wood. For example, power washing should never be used for removing coatings from Western Red Cedar because this process can severely damage the wood surface fibers and make it difficult for the next finish to adhere properly.
Refinishing Opaque Finishes
Western Red Cedar that has been finished with paints or solid-color stains is best refinished with the same type of finish originally applied. These finishes are sometimes used interchangeably but old latex coatings should always be refinished with latex coatings and never with oil-based coatings. Old oil-based finishes can be refinished with latex finishes only when the old oil-based finish has been properly cleaned and a primer coat applied first. Remember, that proper surface preparation and cleaning before refinishing are essential for optimal performance of the new finish coat or coats.
To refinish the old surface, first scrape away all loose, cracked or peeling finish. Sand the bare wood and any remaining finish to “feather” the edges smooth with the bare wood. Mildew must be killed and removed before Western Red Cedar is refinished, or the mildew will grow through the new paint coat or solid-color stain. Removal can be done with a commercial mildew remover or with dilute solutions of liquid household (oxygen based) bleach followed by thorough rinsing with clean water. After these preparations, scrub the surface with a stiff bristle (not wire) brush and water, and rinse with clean water. Allow the washed surface to dry before recoating and apply primer paint to areas of bare wood. After the primer has dried, apply one or two topcoats of paint or solid-color stain. Two topcoats are always better over bare wood that has been primed.
Refinishing Clears, Semi-Transparent Stains and Bleaching Oils
Oil finishes and water-repellent preservatives can be renewed by a simple cleaning of the old surface with a stiff bristle (non-metallic) brush and water followed by an application of a new coat of finish. In some cases, a mild scrubbing with a detergent followed by rinsing with water is appropriate. In more drastic cases, mildew cleaners must be used. The second coat of water-repellent preservative will last longer than the first because more can be applied as it penetrates into small surface checks which open as the wood weathers. The rougher the surface, the more finish can be applied, and the longer the service life.
Semi-transparent oil-based penetrating stains are relatively easy to refinish. Excessive scraping and sanding are not usually required. Simply use a stiff bristle (non-metallic) brush to remove all surface dirt, dust, and loose wood fibers. Following proper cleaning to kill mildew contamination, apply a new coat of stain. The second coat of penetrating stain often lasts longer than the first coat because more can be applied as it penetrates into small surface checks. Note that steel wool and wire brushes should never be used to clean Western Red Cedar. Metal deposits can react with chemicals naturally occurring in the cedar to yield dark blue-black stains on the surface. Weathering stains and bleaching oils are refinished the same way as the semi-transparent oil based penetrating stains.
Semi-transparent latex stains act more like very thin paints and may require more extensive surface preparation (scraping, sanding, etc.) before being refinished. Care should be taken not to build-up the film thickness by recoating too frequently. Manufacturer’s instructions should be followed carefully.
Weathered Western Red Cedar
Uncoated, weathered Western Red Cedar siding or trim can often be restored to its original color by applying commercial products called cleaners, brighteners or restorers. Although intended primarily for restoring horizontal wood surfaces such as decks, these products generally work almost as well on vertical surfaces. Some products are formulated with thickening agents to help the liquid cling better to vertical surfaces.