Improve your results by understanding wood characteristics.
by Kevin Southwick
EACH SPECIES OF WOOD has unique finishing characteristics, both positive and negative. To help you determine how to choose the right wood and get the best results when you finish your next project, I'll explain those characteristics and sum them up in a chart that divides 11 commonly used woods into categories that affect their finished appearance. The notations in each category are based on my observations and experiences with these woods as a professional wood finisher over the past 15 years.
Medium pores throughout
Each species of wood is unique in appearance, thanks mainly to variations between its earlywood and latewood, but also because of the size and distribution of its pores. Pore structure is important when finishing, because most stains accentuate the pores. In ash, pores appear in the earlywood, but not in the latewood, so staining creates a strong contrast between these two elements. In walnut, the pores are evenly distributed across the earlywood and latewood, so staining creates a more uniform appearance. In maple, the pores are so small they're virtually invisible—until stain is applied. Then they appear as dark specks that cover the surface. Also, in oak, walnut and many other species, the pores are large enough to appear as crevices when a clear finish is applied. If a glass-smooth surface is desired, these woods require extra finishing steps to fill the pores.
Fresh Planed Color and Natural Color Change
All types of wood, even finished woods, change color over time, as the result of exposure to air and light. Both the color and the rate of change can vary widely. For example, cherry and maple darken relatively quickly; walnut and mahogany slowly become lighter. Knowing what color the wood will eventually become is important for finishing. It may affect whether or not you decide to use stain, for example. And if you want the Morris chair you're building to look authentic, it's important to know what colors to add to make that new quartersawn white oak look like it's 100 years old.
Sandability and Minimum Final Grit
When is it time to stop sanding? The answer depends on the type of wood and the type of finish. Basically, it's as soon as you can no longer see any sanding scratches. Dense, hard woods with smooth texture and small pores require the most effort and sanding to the highest grits. Woods with large or medium size pores allow stopping at lower grits, because the coarse texture helps to disguise the scratches. The chart indicates the minimum grit at which you can quit sanding for a clear varnish finish. Sand more carefully if you plan to stain the wood—scratches that won't show with a clear finish are likely show up when you stain. Many woodworkers sand to finer grits for oil finishes.
Many types of wood stain well with oil-based pigment stains. The color soaks in readily and evenly and the results look good. However, some woods are difficult to stain dark, due to their density. And some woods are tricky to stain due to blotching, the random, uneven and unattractive absorption of stain. For woods that are difficult to stain dark, apply multiple coats of pigment stain or start with a dark-colored dye stain. Stain controllers or wood conditioners can be used to minimize blotching.
Oil-based and water-based finishes have very different effects on a wood's finished color. Oil-based finishes typically add a slight amber tone that benefits dark colored woods such as cherry, but can give maple and other light colored woods an unwanted yellow tone. Water-based finishes add little to no color, keeping light colored woods looking natural, but leaving dark colored woods looking pale, or even parched. Orange (amber) shellac resembles an oil-based finish. Blonde (clear) shellac and nitrocellulose lacquer add less color than oil-based finishes, but more than water based finishes.
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