Indoors or out, wood is affected by moisture. Wood swells when it gets wet and shrinks as it dries, whether the moisture is liquid water (like rain or dew) or water vapor in the form of high humidity. However, wood coated with the proper finishes will be affected less than wood left unfinished.
Tests conducted at the Forest Products Laboratory (FPL) on the moisture-excluding effectiveness of finishes on wood surfaces show that no one coating applied on wood entirely prevents moisture uptake in high humidity or drying in low humidity. Researchers did find that the moisture-excluding effectiveness of wood finishes varied greatly. Some were very good to excellent, some were poor, and many were in between. The most effective approach was when three coats of finish were applied to the wood surface. Paraffin wax and epoxy paint are very effective; paste wax and linseed oil perform poorly.
Moisture-Excluding Effectiveness (MEE) of Wood Finishes (three coats after 14 days at 90% relative humidity).
|Melted paraffin wax (1 coat, dipped)
|Two-component epoxy/polyamide gloss paint
|Aluminum-pigmented polyurethane gloss varnish
|Soya-tung satin enamel
|Pigmented flat shellac
|Two-component polyurethane wood sealer
|Orange or white shellac
|Phenolic/tung floor sealer
aWood finishes vary in their effectiveness at excluding moisture. Higher numbers indicate greater effectiveness.
Several factors determine how effective a finish will be in controlling moisture. One is film thickness. In general, the more coats applied, the slower the moisture changes and the greater the protection. A second factor is the type of finish used. Pigmented coatings, such as oil-based paints, are usually more effective in retarding moisture changes than clear coatings, such as varnishes and shellacs. A third factor is time. Even good coatings lose their effectiveness over time. The longer the exposure, the lower the effectiveness. Finally, finishes will protect wood from moisture only when applied evenly to all wood surfaces. Unequal coatings on the surfaces of a wood piece may cause unequal shrinkage and lead to warp.
Controlling moisture is very important in using wood indoors and outdoors. The information developed on moisture exclusion should be helpful in determining which finish should be used for a particular need. This information is particularly valuable to furniture finishers or anyone wishing to protect wood from temporary high or low humidity.
U.S. Department of Housing and Urban Development
Forest Products Laboratory