Wood on the Move
- • Wood Moisture
- • Wood Shrinkage I
- • Wood Shrinkage II
- • Finishes
- • Wood Movement
Finishes
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 | ||
| 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. | ||||
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