When you use real wood, extractives are often part of the deal for better or for worse. When you know what kinds of issues to anticipate, you’ll be more effective at compensating for the issues these chemicals can cause in both reclaimed and freshly milled lumber (see Part 1).
Oils in Tropical Lumber Species
While in that first post we primarily focused on sap, that’s not the only troublemaker; many tropical lumber species have a high amount of oils, which aids them in resisting rotting and being repulsive to insects. Like sap, these oils can continue leaching from the surface of the lumber, even after it’s milled, causing trouble when it’s time to apply finish or glue. In order to combat these potential problems, you’ll need to wipe down the boards with a solvent to remove the oil; before it has a chance to return to the surface, you need to apply your finishing product of choice. Careful planning and timing is necessary in order to ensure that your added effort does not need to be repeated.
Staining in Tannin-Rich Lumber Species
Another chemical that tropical lumber species contain are tannins; you may be familiar with these extractives, which are known for their part in aging whiskey and wine. While you may appreciate the way they enrich your beverage of choice, you probably don’t appreciate the way they interact with any iron-based hardware, causing black stains in the areas surrounding nails or screws. One tannin-rich species is Oak. You can avoid staining from tannins by using fasteners and hardware made of stainless steel or coated in powder.
After installation or building, any tannin-rich wood should be sealed using an oil-based primer or alternative finish, in order to keep water from bringing even more tannins to the surface of the lumber. Tannins can still leach from boards when they’re exposed to rain water, which contains all kinds of impurities. When those impurities react with the tannins, they cause color transfer between boards as well as onto a steel substructure. The resulting solution becomes caustic, picking up dirt and other pollutants, indiscriminately dripping a dark liquid stain onto nearby surfaces.
You’ll often notice this kind of staining on slats with square bottoms or on siding, where residual water has remained until evaporation takes place. This kind of staining can be removed only with sanding or planing. To avoid staining in those situations, the use of an edge treatment or separation between materials essentially forces the tannin-rich runoff to drop off the structure instead of running down its surface. You can use washers to step off the metal substrate slats; alternatively, a moulded drip edge can give the bottom of the board a small surface or drip edge that allows for shedding of the water.
Whatever the scenario, the basic issue is that tannins are dissolved by water, which allows them to flow and react with other substances. You can avoid tannin staining by sealing the wood with an oil-based product to block water from reaching the extractives in the first place.
Continue reading with Part 3.