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.
Practical
approach
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.
