FINISHING A TIMBER FRAME
Although
a timber framed structure is often compared to a fine piece of furniture, the
finishing requirements and objectives for heavy timbers and quality furniture
are quite different. The finish itself should be different as well.
Varnishes, lacquers, and urethanes work well on kiln-dried,
dimensionally stable furniture-grade lumber, but these same finishes would be
impractical on timbers, whose fibers and cells must "breathe" and
move, due to varying moisture content and stresses within the wood. Green
timbers seek moisture equilibrium, and the escape of moisture along with the
chemistry of green wood will break down or oxidize a finish that seals the wood
completely. With re-sawn material the same factors occur in reverse: the dry
fibers need moisture and suppleness from the finishing oils to reduce further
checking and brittleness, as new stresses and movement from re-sawing and
re-joining come into play.
For
either green or re-cycled material, there are three goals in the finishing
process. The first is to reduce the checking, drying, and shrinking process in
both the timbers and joinery. The second is to enhance the beauty of the
particular species of wood that is used. At the same time, the finish must be
easy to apply, and simple to maintain and re-finish over the years. Last, but
certainly not least, the finish should be relatively safe to both the users and
the environment.
Most
commercially available finishes cannot adequately accomplish all of these
objectives for several reasons. Nearly all of these finishes contain metallic
driers which speed up the drying time of the finish, necessitating the
application of many thin coats that only serve to seal the surface of the
timber. The wood simply cannot stay wet long enough to allow the oils to be
absorbed into the cells, fibers, and checks of the timber. These driers,
although used in small quantities, are quite toxic. Also, most available
finishes are 70-80% solvent, leaving very little protective material on or in
the timber. Re-application only serves to seal the timber further because of
the rapid polymerization due to the driers. And the higher the solvent ratio,
the higher the VOC (volatile organic content) level in the shop. Some (but not
all) finishes out-gas aldehydes as they dry, although water-based finishes are
much safer in this respect. Typically, touch-up work, reapplication, and
periodic maintenance is a time-consuming process.
A
good finish for timbers must be drier-free in order to allow time for
penetration and absorption, and to prevent rapid breakdown of a finish that
merely seals the surface. (On timbers, finishes that seal the surface only are
more prone to UV degradation, as well to oxidation for the previously mentioned
reasons.) Without driers in the finish, if properly applied, the finish will
definitely reduce checking and shrinking, even in green oak.
Secondly,
a good finish for timbers must be able to be easily and rapidly applied due to
the large surface area of the frame. The excess should be easy to wipe off
without leaving lap marks and a gummy build-up on the surface. The optimum
solvent-to-solids (oils, waxes, resins) ratio is about 50/50. The ingredients
should be the purest grades in order to discolor as little as possible over
time. Such a finish is flexible, "living" in and not on the timber,
and is easily replenished through the years as needed without a lot of surface
preparation. I recommend application of one good heavy coat, filling all
surface checks and coating all joinery, prior to raising; and touch-up and
cleaning after drying in the frame, using more finish and a rough pad where
necessary to remove scuffs and abrasions, dirt, dust, and paint, and of course,
the inevitable footprints. Over the years, as the timbers appear dry and
"thirsty," more finish can be applied as needed to keep the timbers
in the best possible condition for generations to come.
Article by R. Michael Baugh, founder of Land Ark Finishes,
Land Ark Inc., Copper Hill, Virginia.
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