It’s no secret that extractives can play a major part in protecting wood from the elements. Those same extractives can also cause complications when it comes to using the wood in various projects (see Part 1). In this article, we’ll take a look at the effects of two more types of extractives on wood stain and color, and what you should do to minimize any negative results.
How Rot-Resistant Extractives & Tannins can Affect Wood Stain and Color
Tropical woods are extremely rot-resistant, which is one major reason why they’re such a popular choice for exterior projects, such as decking and boardwalks. Bugs often don’t want to eat these woods, because they find the extractives in the wood distasteful. That’s the upside of the high oil content in tropical hardwoods. The downside is that this oil tends to continually weep in the same way that sap does in softwoods. A special solvent must be methodically applied to the surface of tropical hardwoods to combat the leaching of these rot-resistant oils. Otherwise, glue and finish can fail to adhere to the wood properly. They may need to be wiped down repeatedly, especially right before adhesive or finish is applied to the wood’s surfaces.
How to Combat Common Discoloration & Transference Problems Caused by Tannins
Tannins are an extractive found in many kinds of wood. They’re used to help age whiskey and wine. When it comes to wood furniture or flooring projects, however, tannins can cause some major headaches. They interact with iron-based screws and nails in such a way that black stains can show up on the surfaces of the wood surrounding the hardware.
One solution to this problem is to use stainless steel hardware in tannin-rich woods. This type of fastener is already the best choice for outdoor wood projects. Even if you’re working on an indoor project, a tannin-rich wood species requires either completely stainless or at least powder or stainless-coated hardware to avoid the appearance of these unsightly stains. After completing the installation of a painted wood project, be sure to seal your tannin-rich wood with a primer or finish in order to stop more tannins from leaching to the surface of the boards.
Rain falling on wood can bring tannins to the surface of the wood as well. The impurities found in rainwater can cause a phenomenon known as color transference. When transference occurs, color can leach from board to board or onto a substructure. The tannin and water mixture attracts dirt and other substances that will put dark stains on whatever surfaces it happens to drip onto. Obviously, you’ll want to do whatever you can to avoid this kind of aesthetic nightmare scenario.
In cases where the tannin staining has already occurred, you may need to use a plane or sander to get rid of the stains. Staining is common in projects containing slats or siding that have square bottoms. They tend to get stains underneath them showing where the water remained for the greatest length of time before evaporating. To avoid this problem, edge treatments should be included so that the tannin-rich liquid drops off the boards in a neat, clean fashion rather than trickling down the side of the building. This can be accomplished with either a moulded drip edge or with the inclusion of washers that separate the boards from the metal substrate to which they’re attached.
Sealing the wood is a must when it comes to combating the negative impact of tannins. Use a primer or an oil-based clear coat for best results if you’re planning to paint the boards. These sealants inhibit water from ever releasing and reacting with the wood’s extractives in the first place.
In our next article, we’ll take a look at how chromophores react to oxygen and what can be done to mitigate their impact on wood.
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J. Gibson McIlvain Company
The McIlvain family has been immersed in the premium import & domestic lumber industry since 1798. Headquartered just outside of Baltimore, the J. Gibson McIlvain Company is one of the largest U.S. importers of exotic woods. As an active supporter of sustainable lumber practices, the J. Gibson McIlvain Company has provided fine lumber for high profile construction projects worldwide. Call (800) 638-9100 to speak with a J. Gibson McIlvain representative.