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Finishing Challenge:
The "barber pole" effect
Anyone who has experience finishing wood knows of the unique challenges this task can present. One of the more common of these challenges is how to contend with the "barber pole" effect that leaves a dark/light striped appearance across the width of a book matched veneer face.
"Barber Pole" Effect
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Tight Side vs. Loose Side
Every veneer component
in a face has a tight side and a loose side. The tight side of the
veneer is close-grained, smoother, and less porous than the loose
side. Stain applied to the tight side will be distributed more
uniformly because the soundness of the surface enhances even
penetration, but it will penetrate unevenly on the loose side because
of the presence of an unavoidable mechanical characteristic known as
lathe checks. Book matching, required unless otherwise specified on
the higher grade faces, involves turning over every other component to
produce an aesthetically appealing mirror image at the joint. This
means the tight side faces outward on alternating components in a face,
while adjacent components have the loose side out.
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Lathe Checks in Rotary Peeled Veneer
Lathe checks develop on the loose side as veneer is sheared from the log, creating small fissures or checks roughly parallel to the grain. These checks form on the side of the veneer that is in direct contact with the veneer knife as the veneer passes between the knife and the nose bar as it is being cut from the log. This shearing action results in stress on the wood as it is bent away from the log as it crosses the knife. Lathe checks in veneer cut with properly adjusted knives and nose bars should be minimal to insignificant, but they will be there just the same. Because of the lathe checks, there is almost 3 times the surface area on the loose side than the tight side, creating the condition that leads to excessive absorption of finish material and distorted reflection of light, leading to finishes similar to that in the photo.
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So what can we do?
There are some measures that can reduce the barber pole effect. The simplest one from the standpoint of the finisher is to purchase panels with the faces slip matched with the tight side out (SMTSO). This means that components are fed into the slicer without flipping over every other sheet. Slip matched veneer applied to the substrate with the loose side to the glue line and the tight side out will accept finishing material more uniformly, because all components have similar surface characteristics. However, as with any remedy, there is a trade-off. For one thing, the face appearance is somewhat less pleasing as the flow of the grain is staggered rather than mirrored as in book matched veneers. For another, specifying slip match could result in a higher price because of demand.
  Book matched - Slip matched -
mirror image at joints staggered effect at joints
Glue sizing is a long-standing practice of applying a light coat of a very dilute solution of glue and water to a wood surface prior to sanding. The combination of glue and water raises and stiffens any loose fibers and seals the lathe checks so that the first sanding step in the finishing operation removes almost all the "fuzziness" typical of the loose side of the veneer. The tight side will not have absorbed as much of the sizing mixture, so sanding will remove most of it, resulting in a more uniform appearance across the entire panel surface.
Wash coating works in much the same way, except that the mixture combines lacquer or sanding sealer diluted with a compatible solvent. The wash coat is simply sprayed on and allowed to dry before the first sanding in the finishing procedure is done. The wash coat locks down the loose fibers and seals off the open grain of the loose side to the extent that sanding makes the absorption characteristics of the loose side similar to those of the tight side.
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The barber pole effect can be an exasperating experience, but with a good understanding of how it occurs combined with a couple of precautions or extra measures, hopefully we can keep it from causing so much grief!
Regards,
You can find a more lengthy discussion of barber pole in my book, A Complete Guide to Hardwood Plywood and Face Veneer, available from the Hardwood Plywood and Veneer Association, Reston VA, www.hpva.org, 703-435-2900
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