Did you know that all lumber species have various amounts and types of chemicals or extractives that help make the wood weather and rot resistant? Some extractives even make it tasty — for instance, the sap that comes from the Sugar Maple is used to make maple syrup! Some extractives are useful when actually removed from the wood, while others serve an important purpose by remaining part of the wood itself, helping trees resist insects and rotting. However, these same chemicals can cause trouble when it comes to applying finishes and glue. When you understand what kinds of extractives a given species comes with and how those chemicals affect the wood, you’ll be able to anticipate the kinds of hurdles that are bound to come your way as you work with that particular type of wood.
Weeping Sap in Reclaimed Lumber
Some species of softwoods in particular have unusually high amounts of sap as well as resin. When you see these species in their green state, you’ll often see sap flowing freely from them. As the lumber is dried, the sap will harden and sometimes crystalize. As a result, the wood can become brittle as well as extremely hard on cutting tools.
You’ll often come across these issues when you’re working with reclaimed wood, particularly old growth Pine. When you combine decades of exposure and dirt that contains silica, the working properties of the lumber will reflect those changes. The long-hardened sap can quickly eat up your blades as you mill and cut the boards to size. When sap incursions cause brittleness, the boards will be more prone to splitting, particularly when you drill into them. Hardened sap can also cause issues when you try to apply certain finishes, making a primer coat essential.
Weeping Sap in Freshly Milled Lumber
When instead of reclaimed lumber you’re dealing with the fresh stuff, you may notice that even when the lumber has been dried, boards may show residual staining from weeping, even months down the road. This issue comes up because the drying process cannot dry all parts of a board at the same exact rate; interior portions will always have a higher moisture content. As the board is milled, resin that has not yet been set is often released. Submerging a board in water or simply allowing it to heat up can cause sap to flow afresh. Western Red Cedar and Spanish Cedar are often dried at a higher temperature than any other species in order to help avoid the unsightly mess that comes when fresh sap surfaces.
Staining is more troublesome than simply affecting a board’s appearance, though; the sap can also keep glues and finishes from adhering to the lumber properly. In order to avoid these sap-related problems, you need to be sure that your lumber is dried properly. You’ll also want to pre-finish the boards in order to seal in the resin and sap, before you install them.
Typically, weeping only occurs once, though, shortly after planing. You can easily remove any apparent staining by wiping down the boards with mineral spirits.
Continue reading with Part 2.
