I do not know how many times I hear from woodworkers, both large commercial operations and small one-shop operations, who report that a surface was very smooth, but then a few days later, the surface was no longer perfectly flat. Oftentimes, this “un-flat-ness” shows up after a glossy finish is applied, as such a finish will reflect even the slightest imperfections. So what is going on?
There are two possible reasons why the surface changes.
First is that we have uneven moisture within the wood when it is first made, we prepare a perfectly flat surface, and then we have a moisture change. With the moisture change comes uneven shrinking and swelling. This situation arises on a gross scale. That is, a door will warp, a table top will distort, and so on over a long distance -- a foot or more. Obviously, the cure is to get the moisture uniform within the piece prior to manufacturing and then keep the moisture content from changing appreciably. This is not discussed further here.
Second is much more complex. To understand this second cause, we need to go back to the structure of wood. Basically, wood is made of very tiny cells, much like a mini-soda straw, with a length of 3 to 5 mm and a diameter 1/100 of the length. Within the growth ring for a given year, many species have the fibers that are formed in the springtime with very thin walls and somewhat low strength. However, as the growing season progresses, the cells have thicker walls with more strength. Perhaps a prime example is in pine where each annual growth ring has different color due to the variation of the cell walls.
End grain of southern pine shows the distinct light-colored, softer, weaker earlywood and harder, darker, stronger latewood.
Whenever a knife, sawtooth or even sandpaper passes over the surface of wood, the forces generated can actually compress the softer earlywood cells. This can happen when veneering (especially with the pressure bar that is used to prevent lathe checks), sawing (especially with a dull circular blade), planing (especially feed rolls and pressure bar), and sanding (especially with dull sandpaper).
In essence, the knife or blade and sandpaper particle asks itself “Is it easier to cut this fiber off or push it down and out of the way?” Obviously, the more pressure generated, the more likely that considerable compression will develop and the weaker cells will be compressed.
The compressed cells (remember that wood cells are like hollow soda straws), may recover or spring-back slightly toward their original size (much like a gently bent piece of wood will spring-back to flatness when the pressure is removed). However, with high pressure, many cells will stay permanently compressed. Actually, I should say “temporarily compressed” as with exposure to moisture (liquid such as from water in a finish, or vapor from a high humidity), most of the compressed cells will recover or spring-back close to their original size.
However, remember that the growth rings have both hard cells that were not compressed and softer cells that were compressed. So only the softer cells experience spring-back. The net effect is that a flat surface now develops small “hills and valleys” within the growth ring.
This similar effect can be illustrated with flatsawn lumber where the hard cells are pushed down into the softer cells right underneath during machining. With exposure to moisture, the softer cells underneath spring-back, giving a ripple or corrugated surface.
So-called raised grain is due to excessive pressure during machining, compressed softer cells, and then spring-back. In this case, the effect was so severe that the there is an actual separation between the growth rings.
It is impossible to control the veneer or lumber manufacturing process. Plus, with the presence of water in the living tree, it is likely that spring-back will occur during this initial manufacturing. However, excessive pressure at the end of drying or when planing or sanding can indeed cause the defect in such products. Even hand sanders with dull paper (which means lots of hand pressure) can compress the cells. So, to avoid this defect, first make sure that before final sanding all products are exposed to a very brief water misting or stemming to recover any collapse. Then finally sanding needs to be done with the sharpest paper and the lightest pressure possible.