Wood on the Move
Finishes slow moisture absorption To limit the defects caused by natural shrinkage of green wood, lumber producers preshrink it by carefully supervised seasoning and kiln-drying. They, and you, would rather have the wood shrink before it is made into a piece of furniture.
Woodworkers apply finishes to wood because -- despite the drying -- wood will both take on and lose moisture. There isn't a finish, though, that completely blocks moisture from re-entering things made of wood. As you can see in the chart, below, finishes only slow the process.
You can't change wood's tendency to shrink and swell; only plan for it. Design with dimensional change in mind. Use wood dried to the average moisture content it will see in use -- 8 percent indoors and 12-15 percent outdoors. Finally, apply the most moisture-resistant finish you can that's consistent with the piece's intended use, and coat all surfaces.
|Laboratory tests show finish effectivenenss |
in keeping moisture out *
|FINISH TYPE||NO. OF COATS||% OF MOISTURE-EXCLUDING EFFECTIVENESS|
|1 day||7 days||14 days|
|Gloss Enamel Paint||2||91||64||43|
|* Testing by the U.S. Forest Products Laboratory in Madison, Wisconsin, compared the moisture-excluding effectiveness of different types of finishes. Tests were conducted on dry Ponderosa pine boards that were coated, then exposed to the moisture vapor of 90 percent humidity at 80° F for from 1-14 days. the results listed here show how only the most common woodworking finishes of the many tested performed.|
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