Finishing New Wood Surfaces
Exterior wood is finished to provide weather protection and to modify appearance. A whole variety of factors can affect durability of an exterior wood finish. Success and longevity of any type of finish is partially determined by the characteristics of the wood. Exterior finishes are most durable on woods that are stable, naturally durable, and not prone to surface checking. Only two woods commonly available as construction lumber meet these criteria: cedar and redwood. In addition, any wood will perform better if it is free of defects (such as knots), is at the right moisture content (12 to 15%), and the surface is clean and fresh.
Wood such as cedar or redwood can be allowed to weather naturally without any protective finish. The final color will be gray, and some checking, warping, and shrinkage may occur. Two basic types of finish systems are recommended for outside wood: penetrating treatments and film-forming finishes. Penetrating finishes include water-repellent formulations and semi-transparent stains. Film-forming finishing includes solid colored stain and paints; clear varnishes with long-term durability are not available for exterior wood surfaces. Acrylic latex paints and alkyd oil-base paints are available for exterior wood. Match the finish to the intended uses and to your personal preferences. But, in any case, use only top-of-the-line paints; avoid cheap stains or paints for exterior wood.
Use brushes to apply exterior finishes to maximize performance. Apply finishes when the temperature is between 50 and 90 degrees F. Do not allow more than two weeks to elapse between successive coats of paint. Apply a primer and two topcoats of paint; this three-coat system will typically last twice as long as a primer plus one topcoat. For semi-transparent stains, apply the second coat before the first coat is dry; this will maximize penetration which is important for durability.