Grain Filling with Shellac
On more "formal" projects, we often want to produce a finish that is "glass smooth". If the wood used is cherry, maple, birch, or a similar closed grain species this is not a particularly difficult task. The small pores are easily filled with the finish being applied. But, if we are working with oak, or even walnut or mahogany attaining the objective can be more challenging due to the open grain structure of these species.
Typically, when faced with the need to fill the grain we will simply reach
for a can of paste wood-filler. In some cases, particularly when working
with species like walnut or mahogany we will elect to use the "sanding
slurry" method of grain filling. Both of these techniques, however,
have the disadvantage of somewhat obscuring the depth and structure of the
pores. Typically, we tint paste wood filler in order to accentuate the
grain. This technique produces striking results, but the pigment added to
the already opaque paste wood-filler has the effect of filling the grain with a
"solid" material that detracts from the 3-D effect produced by the
physical depth of the pores; we can no longer see to the bottom. The
sanding slurry technique produces the same result, though the obstruction in
this case is sanding dust from the wood being filled. Therefore, the
objective of this technique is not to color the filler; it is, rather, to fill
the grain with material of the same color as the wood (An exception occurs when
we use a tinted oil/varnish blend to produce the sanding slurry). But,
what if we want to fill the grain and at the same time create no visual
obstruction at all; what if we still want to see the grain structure all the way
to the bottom of the pores as though we are peering into miniature canyons that
have been filled to level with water
This objective is easily achieved by applying successive coats of clear
finish and then leveling them by sanding or scraping until the grain is
filled. Any clear topcoat finish can be used in this technique; but
shellac has a distinct advantage in that it dries very quickly thus permitting
the job to be done in a single day. Begin the process by sanding thru 180g or
220g taking care to remove all milling marks, surface defects, and sanding
scratches. A better technique that will produce an even smoother surface
would be to prepare for the finish using a smoothing plane and cabinet scraper,
or hand held card scraper.
Once the surface is prepared, vacuumed it clean and then wipe it down with mineral spirits to "preview" your finish and to check one last time for scratches and missed glue squeeze out, where appropriate. Then, using a freshly mixed 1# cut of shellac in the grade of your choice, apply four to six coats of shellac, allowing each coat to dry before the next is applied. Note that this instruction is in apparent conflict with other instructions to limit the number of coats of shellac applied. However, as we proceed it will become apparent that there really is no conflict. We are using the shellac as a grain filler, not a finish.
When the last coat shellac is dry (25 to 30-minutes will be sufficient drying time for each coat if the shellac is fresh) sand or scrape the just applied shellac back to bare wood. If you elect to sand, use a backer block and 150g to 180g open coat sandpaper. Do not fail to use a backer block; it is vital if you are to obtain a flat surface. Scrape or sand until the shellac you just applied is removed from the surface and remains only in the pores. When you are finished the surface should appear "freckled" with pinhead size shinny spots of shellac in the recesses of the pores, but none on the surface. At this point you will have successfully filled the grain on all but the most coarse textured species. Clean the surface again with a shop towel lightly dampened with mineral spirits to remove the dust. Then, after the mineral spirits has evaporated, apply four to six additional coat of shellac . (Again note that the multiple coats of shellac are being applied to bare wood as a grain filler, not as a finish.) When the shellac is dry scrape or sand as before until the shellac is uniformly dull (no shinny spots indicating unfilled pores). Take the shellac down only until you just begin to strike bare wood. If you notice any "freckles", or pinhead sized shinny spots within the dull areas of sanded shellac it will be an indication that the pores in the wood grain have still not been filled even with the surface. In that case clean the surface as before and apply two or three more coats and scrape or sand them down. Repeat until you achieve a uniformly dull surface with no shinny spots indicating that the pores are completely filled. In most cases, if the wood is properly surfaced, the pores will be completely filled after the second set of four to six coats of shellac.
Let's pause here for a "reality check". The first set of four to six coats, applied and scraped or sanded back as described, will give you a vivid illustration of just how well you have prepared the surfaces—how "flat" it is. If you have done a good job and the surface is truly flat, you will cut through the shellac to bare wood everywhere at once. When cutting through the second set the entire surface will be "dulled" at about the same time (though you may will still see a pattern of 'freckles' depending on the texture of the wood).
On the other hand, if you expose bare wood in an alternating and irregular patters of shellac and bare wood it is a positive indication that the surface is not level. For example, if the pattern of bare spots and dull shellac are aligned in more or less parallel rows it is an indication that your work to this point has left "waves" on the surface of the wood. This could indicate residual planer marks if the bands are narrow and closely spaced. If they are further apart and aligned with glue-ups, look to how you are sanding your glue-ups, particularly if you are using a belt-sander—you are probably lingering over the glue seams and not sufficiently sanding the spaces between. A pattern of bare wood that is composed of more or less circular spots randomly distributed over the surface indicates that you have created a series of 'valleys' and "hills", probably by improperly sanding the surface with a random orbital sander to remove 'spot defects'.
If you do get a strong indication that the surface is not level you may want to stop the filling process here and return to preparing a level surface—continuing to work with a 'irregular' surface will defeat the purpose of filling the grain to achieve a glass smooth surface in the first place. When the surface is uniformly dulled with no shinny freckles the grain is filled, clean the surface one last time with a shop towel dampened with mineral spirits. You are now ready for your topcoat. When the finish is complete the surface will be glass smooth and, since there is nothing in the grain but optically clear shellac, you will be able to see unobstructed to the bottom of the pores. The topcoat can be varnish, lacquer, or a water-borne finish. It can also be shellac padded to a high luster.
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