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Wood on the Move

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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 *
1 day 7 days 14 days
Tung Oil 2 46 2 0
Lacquer 2 70 22 8
Shellac 2 84 43 20
Spar Varnish 2 80 36 15
Urethan Varnish 2 83 43 23
Gloss Enamel Paint 2 91 64 43
Polyurethane Varnish 2 90 66 46
Two-Part Epoxy 2 98 93 88
* 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.

Continued on page 5:  Wood Movement


Comments (2)
ralphcd wrote:

What would be a way to keep moisture in wood supplies in a dry climate like Las Vegas?

11/13/2014 10:13:47 AM Report Abuse
wolflahti wrote:

This is reversed in the Pacific Northwet (not a typo), where winters are far more humid┐indoors and out┐than the summer season (which doesn't last nearly so long).

11/6/2014 01:26:55 PM Report Abuse

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