No matter what anyone tells you, wood will not "drink up" the oils and restore life to your siding or roof. The wood is dead.
Water-repellent preservatives may be used as a natural finish for wood surfaces. They contain a wood preservative, a small amount of wax as a water repellent, a resin or drying oil, and a solvent such as turpentine or mineral spirits. Water-repellent preservatives do not contain any coloring pigments. Therefore, the resulting finish will vary in color depending upon the kind of wood. The preservative may prevent wood from graying by inhibiting mildew.
Water repellent preservatives may also be used as a treatment for bare wood before priming and painting or in areas where old paint has peeled, exposing bare wood. This treatment keeps rain or dew from penetrating into the wood, especially at joints and end grain, and thus decreases the shrinking and swelling of wood. As a result, less stress is placed on the paint film, and its service life is extended. This stability is achieved by the small amount of wax present in water-repellent preservatives. The fungicide inhibits surface decay.
Be sure to purchase the correct type of water repellent preservative. Any type of water repellent preservative can be used as a natural exterior finish by itself, but only some are paintable. Manufacturers have also developed water repellent preservatives specifically for exterior finishes.
Water repellents are also available. These are simply water repellent preservatives with the preservative left out. Water repellents are not good natural finishes but can be used as a stabilizing treatment before priming and painting. Before purchasing and using a water-repellent preservative or water repellent, read the label carefully and follow the manufacturer's directions.
The most effective method of applying a water repellent preservative is to dip the entire board into the solution. However, brush treatment is also effective. When wood is treated in place, liberal amounts of the solution should be applied to all lap and butt joints, edges and ends of boards and panels. It is important to apply liberal amounts of the solution to the end grain of wood. Areas especially vulnerable to moisture, such as the bottoms of doors and window frames, should not be overlooked. One gallon will cover about 250 square feet of smooth surface or 150 square feet of rough surface. The life expectancy is only 1-2 years, depending upon the wood and exposure. Treatments on rough surfaces are generally longer-lived than those on smooth surfaces. Repeated brush treatment to the point of refusal will enhance durability and performance.
Water repellent preservatives can be renewed by a simple cleaning of the old surface with a bristle brush and an application of a new coat of finish. To determine if a water-repellent preservative has lost its effectiveness, splash a small quantity of water against the wood surface. If the water beads up and runs off the surface, the treatment is still effective. If the water soaks in, the wood needs to be refinished. Refinishing is also required when the wood surface shows signs of graying.
Note. Steel wool and wire brushes should not be used to clean surfaces to be finished with water repellent preservatives since small iron deposits may be left behind. Pentachlorophenol may cause iron remaining on the surface to corrode. The corrosion products may then react with certain wood extractives to form a dark blue, unsightly discoloration which becomes sealed beneath the new finishing system. Pentachlorophenol was commonly used in some semitransparent penetrating stains and water repellent preservatives before it became a restricted use pesticide.
Water repellent preservatives can also be used as a natural finish for plywood. Water-repellent preservatives are mixtures of a solvent such as mineral spirits or other paint thinners, wax, a resin or drying oil and a wood preservative. These finishes, like semitransparent stains, penetrate the wood and do not form a surface film, so peeling will not be a problem. Since they do not contain any coloring pigments, they will allow he natural wood color and grain to show through. Expected service life is only 1 to 2 years, and frequent reapplication is necessary to protect the wood surface.
Water repellents are sometimes used in the same manner as water repellent preservatives. However, they do not contain a wood preservative and will not protect against surface mold and mildew.
Wood preservatives are not considered to be finishes. However, wood properly treated with a preservative can withstand years of exposure to severe decay and insect attack without being affected. The common wood preservatives are creosote, penta-chlorophenol in oil, and copper and sodium napthanates. The newer water-borne salt treatments, all of which are restricted use pesticides. Creosote and penta-chlorophenol in oil result in a dark and oily surface. Odor with creosote is a problem. Wood treated with creosote or penta-chlorophenol in oil is not recommended for use around the home where people will come in contact with it. However, wood treated with water borne salts is suggested for use as patio decks, outside steps, privacy fences and other home uses. This material is generally light to bright green or brown in color. It can be used outdoors without finishing and will go practically unchanged or weather to a light gray.
This article has been contributed in part by Michigan State University Extension