Q. From time to time we experience grain raising. We are not sure what causes this or what we can do to minimize or avoid this issue. Any guidance is appreciated.
A. This is indeed a complex question. I will try to be specific, but if you have more questions after reading the details here, please let me know. A picture of what you are calling grain raising would also help me be more specific, as this term applies to several different situations, as you will read about in the following paragraphs.
First, the term itself is a bit misleading. That is, we might wonder how the grain of the wood is raised. In truth, it is not the grain of the wood that is raised, but individual cells or fibers in or at the wood’s surface that are raised. So, if this is true, which it is, then let’s back up to try to get an understanding of what wood is all about. (Did you ever wonder why grain raising occurs only on the side grain and not the end grain? I am going to tell you the answer to this as well as the answer to why we get grain raising.)
What is wood?
To help understand this material we call wood, it is a very close analogy to consider wood as a bunch of miniature soda straws running parallel to each other and glued together. How miniature? The typical wood cell (or fiber or soda straw) is about 3 mm long; the needle trees that we call softwoods are 3 to 5 mm, while the leaf trees that we call hardwoods are 1 to 3 mm. The diameter of the typical cell is 1/100 of its length.
These wood cells are tiny indeed. In fact, a 1-inch cube of wood contains about 5 million cells or fibers (or miniature soda straws). Most of these cells run vertically in the tree with their main job being the transport of liquid from the roots to the leaves. A few cells that are called ray cells store starches and sugars run horizontally. All these cells are “glued” together by a chemical called lignin. Lignin is also the stiffener for the cells, giving them their strength.
With this image or analogy in mind, we can understand why end grain is so porous; it is basically the ends of millions of miniature soda straws. Thin liquids put on the end grain quickly penetrated down the open pores of the cells; thick liquids cannot penetrate these miniature holes. To help penetration into small openings, a surfactant is often added to water to destroy the water’s surface tension. A chemical like alcohol has little surface tension, so it penetrates really well.
So much for end grain, although a quick note about machining end grain. Can you imagine how hard it is to cut a bundle of straws cutting off the ends? The straws will collapse and deform and do more tearing than clean cutting, especially with a dull knife. The same with end grain machining…hard to do and achieve a premium surface.
So now let’s consider the side of this bundle of miniature soda straws -- the side of these wood cells. When we machine the sides, we are actually tearing the walls apart or tearing one cell from its adjacent cells. Sometimes, we tear just part of the cell off and leave the remnants still fastened to the wood.
In addition, sometimes when machining, especially with a dull knife, with slow feed speeds (i.e., many knife cuts per inch), at high moisture contents, with lower density species (lower lignin content) or with wood fibers that are not as strong as “normal” such as with tension wood in hardwood species, the knife finds that it is easier to push the fiber out of the way (push the fiber into the final surface) or squish the fiber down into the surface), rather than cut the fiber off.
A special note is that sanding is a machining process. The fine, sharp particles of the sandpaper do exactly what a knife does: cut fibers, push fibers, tear fibers.
Finally, as part of our analogy, consider what might happen if there is a lot of side pressure on this bundle of wood cells. With a lower density, a weaker and more easily compressed piece of wood, the machining tools, including hand sanders with dull sandpaper, will actually cause some of these hollow soda straws (cells or fibers) several layers deep, even several layers away from the surface, to become compressed or squished. Even so, the finished, machining surface will look perfect initially, and probably look perfect for days and years to come if we do not touch or affect the surface in any way.
Effects of water
So, now comes the bad guy -- water. This water can be vapor (high humidity) or liquid, although with liquid the effects are much faster and more dramatic.
When water contacts a loose, squished or compressed cell, this cell springs back to its original shape (or at least tries to reach its original shape). What this means is that the individual fiber will pop back (sometimes called springback) to its original size and shape. Fragments of a fiber will often move too. Loose fibers will pop back up. As these are individual fibers, or parts of fibers, the overall effect will be a fine peach-fuzz appearance.
The extent of this raised fibers (not really raised grain) depends on how many have been damaged in previous machining or sanding (i.e., there can be a lot or just a few) and the amount of moisture added. When finishing, these fibers that stick up through the finish are what many people call grain raising. Note that it is hard to sand these errant fibers away; they like to flop back over. We can use a stiffener (glue-sizing or sanding sealer) prior to final sanding that will help somewhat. Or we can re-sand very lightly after the first step of finishing…forget this. as it sounds too expensive.
Additionally, dry wood exposed to higher humidity or exposed to water in a water-based finishing system will develop a non-smooth surface, especially if the compressed areas are fairly large. We can see this non-smooth surface most dramatically with a glossy finish.
If we have two pieces of wood that are glued together, the glue line itself is usually more rigid than the rest of the wood, so we can also see the glue line joints more clearly as slight depressions. After the water dries, sometimes these glue lines appear as being slightly raised.
In the extreme if we have a machined, flat grain piece of wood that had a bit too much pressure or pounding from the knives during machining, the joints between the individual growth rings can be weakened, so that the exposure to moisture, which causes stress, can result in failure. In this case, the grain (not just a few fibers) not only raises, but separates, a defect which we sometimes call shelling.
What to do
The use of water-based finishes puts a lot of stress on the wood. If machining was not quite perfect (dull knives or sandpaper, excessive pounding or pressure, and so on), the cells at and near the wood’s surface can move when moisture us added, potentially creating raised grain, uneven surfaces and separated grain. Bluntly, more attention to achieve perfect machining can improve the finished appearance.