Carol Clausen and Samuel Glass tell us about another easy and inexpensive trick for preserving your building projects in Build Green: Your Wood Can Last for Centuries. Coatings that form a moisture-impervious film are effective in preventing decay if the seal where two pieces of wood are joined is kept intact. A well-maintained film of paint over the wood and good paint seals in the joints shed water, thus protecting wood by keeping it dry.
Cracked paint seals permit entry of water and contribute to decay. Water seeping through the broken seal where wood is joined together will be rapidly absorbed at the ends of the wood pieces; its loss by evaporation from the sides of the wood member is retarded by the impervious, intact paint film over the rest of the wood. This keeps the wood moist and favors decay, at least near the joints. The effectiveness of a moisture-impervious paint in protecting wood from decay depends upon the quality of the seal in the joints.