Stains and extractives affect lumber color — and so much more! The more we learn about the natural qualities of lumber, the more effectively we can all work with this fabulous green building product.
Added Effects of Sap
In addition to its unattractive appearance which we discussed in Part 1, sap can cause problems for glues and finishes. As a result, sap spots or signs of weeping need to be removed before a finish or glue can be successfully applied. Once wood with a high sap content is dried correctly, it will require pre-finishing in order to seal in the sap before the boards can be installed; this added step is especially important if the wood will be getting extra sun exposure.
Typically, any added weeping occurs only once and usually soon after planing of the board. Naphtha or mineral spirits can be used to remove the sap. After wiping down the board, simply set it aside and watch to make sure it doesn’t weep more. You should also realize that although it’s rare, occasionally weeping sap can surface decades or even centuries after installation, particularly when boards once used for exterior applications are used for interior ones.
Similar Issues with Oil
Typically, sap isn’t the only issue that contributes to a given species’ resistance to finish and glue. Many tropical hardwood lumber species resist rotting, thanks to their high oil content (and resulting terrible taste in the mouths of bugs that would otherwise try to eat the wood). The oil behaves similarly to how sap would in softwoods with the liquid leaching from the surface, causing issues for glue and finish. By wiping down the wood with a solvent, the oil can be removed; as long as the finish or glue is applied soon after, it should adhere to the surface. Be warned, though, that the oil will continue to come to the surface.
Discoloration from Tannins
In addition to sap, tannins represent another category of extractives. Well known for their ability to age whiskey and wine, these naturally occurring chemicals are known to create black stains when they react with the iron in hardware. (You’ve probably noticed this issue surrounding screws and nails.) Oak is a species that’s particularly rich in tannins and therefore especially prone to this type of discoloration. Some exotic species also include high amounts of tannins, making them subject to similar staining. For exterior applications, stainless steel fasteners are recommended, and these won’t react with the tannins in the wood.
Whenever you’re using hardware on a species that’s particularly rich in tannins, using hardware that’s either stainless or powder coated will help eliminate the issue of staining. After that, sealing the wood with an oil-based primer or finish will help block the water from dissolving added tannins and potentially causing staining to occur.
Continue reading with Part 3.