How We Apply Our Oil Finish

Anticipation precedes and delight accompanies the application of an oil finish to raw wood well sanded.  The first flow of oil enriches the color, enlivens the grain, and ignites a fire within.  To achieve an excellent finish, however, considerable time, work, and trouble must follow that first transformation.

The Material

We finish with Deft Danish Oil to which we have added Deft Polyurethane Varnish (Defthane), about 20% by volume.  The addition of the varnish adds more resin to the mixture, therefore increasing the rate at which the finish builds, but also reducing the margin of error and creating new opportunities for making a mess.  The resins will form the final finish.  In these Deft products, they are derived from natural sources (tung oil and linseed oil) and synthetic ones (urethane).  When applied, the mixture soaks into the top layers of the wood fibers.  The actual depth to which it penetrates depends upon the species of wood and the care given to application, but is never over .04".

A molecule of resin may be described (imprecisely) as a short chain of linked carbon atoms attended by units of hydrogen and oxygen.  As drying occurs these chains link up (polymerize) into large, matted, complex molecules entwined within and bonded to the wood fibers.  Subsequent coats of oil add to this structure.  The finish is built coat by coat from as deep as it will penetrate up to and upon the well-sanded surface.  As more resin is added and polymerized into the thickening web, the finish becomes tougher and more water resistant, and the reflection of light from the surface to the eye becomes sublime.

Surface Preparation

Sanding.  I'll try to be brief.  Nothing about the reality of it is brief.  We use many different sanding grits, machines, and techniques, depending on the part and profile.  The object of all prior machine sanding is to produce a surface on an assembled rocking chair, dining chair, or other piece of furniture that can be expeditiously hand sanded with 320 grit.  No tool marks or coarser sanding scratches should survive this hand sanding.  We then hand sand with 400 grit, looking carefully for any irregularity we might have missed.  The quality of luster that an oil finish develops is directly related to the size of the scratches left by the abrasives.  Because light is reflected more evenly (less scattered) by a smooth surface than a coarse one, the finer the scratches are, the finer the luster is.  We have found that a final raw wood sanding with 400 grit followed by sanding with 600 and 1000 grit between coats of finish produces a pleasing sheen without glare and a sensuous surface to touch.

Applying the Oil

The objectives for each application of oil are to satisfy the porosity of the wood by keeping the surface wet with finish and rubbing and then to remove all excess finish by wiping with a dry cloth.  Timing and technique are important.  If the surface is not kept wet long enough, penetration suffers.  If the excess finish on the surface begins to become sticky, the finisher suffers.  The labor to remove the excess increases drastically as the resins begin to polymerize.

To apply the oil, we wear rubber gloves and work in an open air space.  We dip a sponge into a bowl of finish.  For the first oiling of raw wood, the sponge should be brought to the wood as wet as possible without causing finish to run down one's arm.  We flood oil over the piece, dipping for more oil as necessary to keep the sponge wet.  The process for subsequent coats is the same except that, because less oil will ultimately be absorbed, the sponge need not be kept as wet.  When the chair or other piece has been coated thoroughly, we continue rubbing, adding enough oil to keep the sponge lubricated.  Some areas of the wood will absorb the finish and will appear dry.  The finish will puddle on other areas.  Continuing to rub evens the finish from puddle to dry and insures that those areas that are most absorbent will receive adequate oil.  After 20 minutes or so, it is time to grab a wiping cloth and begin to wipe off the finish that has not been absorbed.  All the excess must be removed.

Liquid finish will sometimes bleed out of joints, inside corners, intersections and wood pores.  Even after the most conscientious wiping, small puddles can appear in these problem areas.  Bleeding usually begins 10-30 minutes after wiping and can continue for two hours or more.  It is unpredictable and exasperating.  The attack against "bleed out" is two pronged.

First, we inject compressed air into the joints and intersections, blowing any extra finish out on to the surface where it can be wiped.

Second, we wipe each piece again after it has dried an hour or so.

We pay careful attention to the areas we have blown out and look closely for small spots of finish that may have bled out of random pores here and there.  If these puddles or spots are allowed to dry, they leave noticeably shiny places that are difficult to remove.

During assembly of the furniture, we remove the excess glue that has squeezed out of the joints by wiping with a wet cloth and scrubbing with a toothbrush.  Any grain raised by this process is smoothed by the 320 sanding.  When the piece is oiled, any glue that is missed by the washing and sanding will show up as a light colored, sealed-off area.  We look for these glue spots and wet sand them with oil and 400 grit paper until they disappear.  Occasionally, we find a bead of glue that was missed by the washing.  These beads will need to be chiseled off before wet sanding the area.

For those interested — view the finish schedule that I have posted to guide us through the finishing process on our rocking chairs and dining chairs.