— Betty Lou, Salisbury, MD
A: Tom Silva replies: A wood preservative contains chemicals that kill the fungi and insects that discolor or destroy wood. Wood preservatives are classified as pesticides, so their use is strictly regulated by environmental laws. They should be handled and disposed of with care, following the instructions on the label.
A water repellent is a penetrating wood finish loaded with oils or waxes designed to prevent water from soaking into wood. Organisms that discolor or degrade wood need water to survive, and a water-repellent treatment makes it harder for them to grow, as long as it is reapplied to bare wood every year or two. Water repellents also stabilize wood somewhat, reducing its tendency to check (crack) and warp.
Water repellents that contain a small amount of wood preservative are called, not surprisingly, water-repellent wood preservatives. Although it's easy to think of these products as being primarily for decking and siding that won't receive a top coat of paint, applying a water-repellent wood preservative to all sides of a piece of exterior wood before painting provides the best protection against decay and doesn't need to be reapplied until the paint fails.