Wood finishing: why does oak stain better than pine?
Posted in Home Improvement on June 27, 2018
Everyone who's dipped a rag into a can of stain can recall the time when rubbing the color into the wood resulted in disaster. Remember? Rather than contrasting hues and lines revealing the subtle beauty of the wood, you found yourself staring at muddy tones, ugly splotches, and glaring surface flaws that made it look awful.
It's a painful reminder that some types of wood (pine, birch and maple, for example) don't take stain as easily as others (oak, ash, hickory). Each wood has its unique character, so if you want your stained pine to look like stained oak, you‘re going to be disappointed.
Yet all types of wood will take a good finish —— some are just tougher to stain than others. In this article you’ll understand why some wood takes stain poorly and what you can do about it.
What is stain?
“Stain” in a broad sense can mean anything you use to color wood, but I'll use it here to mean “pigmented” stains, because that’s what you'll most often find in a home center or paint store.
Pigmented stains are the easiest to apply. They’re actually a thin paint, consisting of a pigment, a binder, and a solvent.
The pigment is a finely ground colored powder, actually a dust that you spread over the surface of the wood to color it. It doesn't dissolve.
The binder glues the pigment to the wood.
The solvent (usually paint thinner or water) liquefies the binder and holds the pigment in suspension so you can spread it evenly. Then the solvent evaporates.
When you spread a stain, the pigment collects and builds up in every crack, crevasse and depression in the surface of the wood.
How does wood absorb stain?
Here's where the nature of the wood itself comes in. Wood consists of thousands of hollow cells (Fig. A), some large and some small. Large cells collect more pigment and go dark; smaller cells collect less and appear light. So when you spread your stain, you're actually highlighting the varying cell structure of a tree.
Relatively large cells in the earlywood of pine absorb more stain and turn dark. Smaller latewood cells absorb less and remain lighter. Blotchy appearance is common to pine and other softwoods.
As you might guess, the cell structure of pine (Fig. A), which takes stain poorly, is quite different from oak (Fig. B), which takes stain well. Most of the cells in pine grow relatively large during the first part of each growing season. The wood made up of these large cells is called “earlywood.” It absorbs a lot of stain compared to the narrow bands of very small cells that form later in the season. called “latewood”.
Though you can‘t see either of these types of cells with your naked eye, stain will color the earlywood darker than the latewood.
ln oak, on the other hand. you can actually see one large type of earlywood cell, called a “vessel.” If you look at the smoothly cut end grain of oak (at the top end of Fig. B) you can see rows of these vessels that look like tiny pinholes to the naked eye. They absorb a lot of stain, in sharp contrast to the microscopic cells in the latewood, which absorb little. That contrast. of course. Is why dramatic “grain” lines show up in oak when it's stained.
Narrow bands of large vessels in the earlywood of oak absorb lots of stain and become darker. Smaller vessels in the latewood absorb less and remain lighter. However, many hardwoods do not have clear bands of vessels and do not take stain easily.
All softwoods (coniferous trees like fir, spruce, cedar, etc.) have a cell structure like pine, and often look bland and lifeless when you stain them.
One reason for this is that most softwoods do not have prominent bands of latewood, so there is little pattern or grain to highlight. Two notable exceptions are Southern yellow pine and Douglas fir. Both have wider bands of latewood that can cause headaches because they contrast too much! (Douglas fir plywood displays the earlywood/latewood contrast most vividly.)
A second reason for staining difficulty is that earlywood often doesn’t absorb stain evenly. Wood cells grow like a bundle of soda straws. Where their ends show on the surface of a board (end grain), they absorb a lot of stain, but where their sides are exposed, they absorb less. Unfortunately. most of the time the wood cells don’t grow together in a perfect bundle, straight up the trunk of the tree. Sometimes they swirl, especially around knots, and emerge on the surface of a board at different angles. These areas will absorb stain differently than the surrounding cells, causing a blotchy appearance (Fig. A).
Third, softwoods stain poorly because they're soft. Their cell walls bend and crush easily as you prepare the surfaces for finishing. So scrapes, dents, and even sanding marks can suddenly appear when you apply a stain.
And fourth, stain reverses the natural contrast in softwoods. Latewood is slightly darker than earlywood, a feature that a clear finish (no pigment) can highlight. But when stained, earlywood absorbs more pigment than latewood, appears darker, and reverses its natural contrast (Fig. A). This might come as an unpleasant surprise if you don't expect it.
Hardwoods (deciduous or broad-leaved trees) aren’t all easy to stain either, despite the fact that they all have relatively large vessel cells (Fig. B) like oak. As mentioned earlier. oak has bands of vessels in its earlywood that absorb stain readily and contrast with the latewood.
However, vessels in many other hardwoods do not occur in bands; often they spread throughout the tree's annual ring. So when they absorb stain, they don't highlight any grain pattern, just like many softwoods. Maple and birch are two common examples, they’re as hard to stain as pine.
What you can do
Just because the cells seem stacked against you when you try to stain some types of wood doesn’t mean your efforts are doomed to fail. It simply means you’ll have to prepare your wood more carefully and experiment with finishes.
Here are ways to improve your chances for success.
If possible, select wood that has good staining potential, particularly hardwoods that show clear bands of vessels and softwoods that have a clearly visible earlywood/latewood contrast. Staining will emphasize the contrast. In softwoods, you can also look for boards with narrow, relatively straight grain lines, where the darker bands of late wood are close together. These boards often stain more evenly than wood containing wide bands of earlywood.
In both hardwoods and softwoods, cells emerge differently on each side of the same board, so examine the grain lines and contrast most closely on the sides that you want to make prominent.
Because softwood surfaces are fragile, fuss over them a bit. Carefully sand out all dents and scratches, glue marks, old finish, etc., ending up by completely hand-sanding the entire surface with fine-grit (120 or finer) paper. Use the same hand pressure everywhere: pressing harder in one area can change the surface texture. (Even if you use a power sander, finish up by hand.) Take the same care with maple and birch, too. Patience and thorough work at this point will pay off in the end.
You can test your surface for flaws before you stain by rubbing it with a cloth dampened in mineral spirits. This will darken the surface just enough to show mars and sanding marks. After the surface dries in 10 to 20 minutes, sand out any flaws, then go ahead and stain.
Always stain an obscure area first, or use a test board of the same material, to see how the wood absorbs the pigment. If you don’t like it, you can always sand it off before you go too far. Fortunately, the pigments in stains don't sink far below the woods surface.
Try other techniques
If your wood still doesn't stain nicely, you have several options.
First, try applying a “sealer.” This is essentially a clear binder (paint without the pigment) that fills, or partially fills, the open wood cells. It keeps those cells from absorbing as much pigment. In effect, it reduces contrast between the light and dark areas. You can choose among many kinds of sealers, including thinned varnish, linseed oil, and commercially prepared stain retarders.
Experimentation is the watchword here, because wood varies greatly, not only among species, but among individual trees and even between different sides of the same board. For example, end grain, as we mentioned earlier, has many deep cells, which absorb a lot of stain. It will always go dark, so you might want to seal it to keep it light.
Second, on a wood scrap, try applying different concentrations of stain for lighter and deeper color. Often light-grained woods look good with a light stain, just enough to emphasize their grain lines and other patterns, but not enough to heighten the blotchy effect caused by uneven earlywood cells. In fact, a clear finish alone might do the job on maple and pine, since it will slightly darken the wood surface and bring out some patterns.
At the other extreme, a very dark stain can veil some of the blotchy effects in some types of wood. The woodworkers who stained the birch doors and trim in my home shaded the blotchy areas darker while they left areas with nice grain lines slightly lighter to emphasize them. It was an effective solution for a difficult wood.
Third, experiment with different brands of stain. Walnut stain “Brand A” won’t necessarily look the same on the wood as walnut stain “Brand B,” even though the color might look the same in the can. Manufacturers often use different pigments, binders, and solvents.
Venture into dyes
A fourth option is to abandon stains altogether and try a dye. As mentioned earlier, I'm using “stain” to refer only to pigmented stains. Unlike pigments, dye doesn't collect in voids in the wood‘s surface Instead, it penetrates and colors the woods cell walls themselves, highlighting the cell structure even more clearly.
That’s why fine furniture makers use dyes: they’re transparent and show off the patterns in wood better, of course, they also show off every irregularity and flaw, too. So until you experiment with a dye, you won't know if it will work any better than a stain.
Look for wood dyes in specialty woodworking shops. They’re harder to apply than stains, so ask for some free advice while you're shopping. Whether they come powdered or in concentrate form, you'll have to mix and apply them to test boards until you find a pleasing color and effect.
As you experiment, you'll probably find that wood finishing is more art than science. At least that's one way to keep positive in the midst of a frustrating wood-finishing job! And knowing that beauty is at least partly in the eye of the beholder can give you a creative edge. (“Creative” solutions look better if your spouse agrees.) Who would have guessed that stained pine — knots, blotches and all — would become as fashionable and popular as it has?
Of course, in the end you might be dissatisfied with everything you try. That’s why most of us paint those hard-to-stain softwoods like pine, and put a clear finish on others like maple. Often there’s little more we can do to make them look better.