The combined weathering action of sunlight and water quickly changes the surface of woods. Any wood changes color and becomes rough. The wood slowly wears away at a rate of about 1/4 inch per century for a common softwood like pine or redwood. Staining microorganisms (fungi) are the main causes of the gray color of weathered wood. Weathering is not associated with decay (rot) but is caused by chemical changes in some of the wood's components. Water repellents and water-repellent preservatives can be used to slow down the natural weathering process, but the treatments will only last 1 to 3 years before refinishing will be required. Weathering can be prevented by using opaque finishes, such as paint and solid-color stains, which screen out the ultraviolet light of the sun. Natural weathering of wood is associated with early American tradition and can be considered as the first method of wood finishing. For the first century or so of the American colonies, exterior surfaces were left to weather naturally. Only later were painted surfaces used by the general populace. Recent interest in colonial traditions and furnishings and the do-it-yourself trend has revived the popularity of naturally weathered wood and rustic finishes. Some wood houses are left unfinished to weather naturally and have lasted for centuries. Wood's aesthetic appeal, life expectancy, and future compatibility with potential finishes is greatly affected by the weathering process. -- CAL