CLOSED-PORE FINISHING
FILLING OPEN GRAINED WOODS
By Erick Kasner, PhD
Woods such as mahogany, rosewood, walnut, chestnut, oak and ash, among others have large
pores in their surfaces. As such, they are referred to as open-grained woods. To achieve a smooth-asglass-surface, though not necessarily glossy, the grain’s cavities must be filled and become flushed even
with the ridges of the grain. That is particularly warranted on mahogany, walnut and rosewood; rarely
done on oak, ash or chestnut. Otherwise, when top coating open grained woods the finish will “drop”
into the cavities of the grain through gravity and/or grain absorption resulting in the finish having that
“hungry look” which may not be appealing to the customer.
Closed pore finish filling is a matter of preference to the finisher or refinisher, as is style or, as
dictated by the client. Many customers would prefer a natural open-pored look where, the texture of the
wood shows through, particularly with oak, ash or chestnut. It is quite attractive in rubbed-effect satin or
a matte finish. For a glossy but more so high gloss, smooth-as-glass finished wood surface, most woods
should be grain filled, including oak and ash, so that no grain should be felt when running your hand
across that surface. With woods having a small, or tight grain, such as cherry, birch and maple, grain
filling can be an easier task. On the more porous woods, and specifically on oak and ash, and over a dark
stain on mahogany or walnut, grain filling becomes a more difficult and a time consuming task – but
does not have to be!
This article deals with pore or grain filling (used hereafter interchangeably), as a function of the
type of wood being filled, and the reasons why to use or not to use a particular method. But, equally
important, what can you do to increase productivity without detrimental consequences down the road,
such as, crazing, cracking or dropping to the finish. It should be prefaced however, with the fact that, as
of this writing, no “miracle” pore/grain filler exist on the market that will magically filled the grain and,
is universally suitable across all grain filling demands. Equally significant, each method has its
advantages and disadvantages, where the operator must exploit its pluses while minimizing its minus to
best suit the needs.
There are four popular methods for pore filling: use of high-solids sanding sealer as grain filler;
use of a high-solids topcoat; a lacquer-based quick-drying grain filler, and conventional paste wood
filler. Each method of pore filling is quite functional, though constrained by productivity, type of wood
grain being filled, the end result sought in terms of desired look and feel, and revenue to be received for
the finished job. That is, properly pore filling with conventional paste wood filler yields the best results smoothest finish, with best clarity and depth, and minimum amount of shrinkage. However, filling pores
with a conventional paste wood filler can be time consuming and laborious, as such least productive of
the four. Though can be very time consuming and hard to sand when used and applied improperly, we
will show you herein how to better, easier and more productively grain fill with conventional (oil based)
grain filler and still get that Steinway Piano look typically obtained through excessive work.
Table 1 at the end of the article summarizes each method to grain filling as discussed below,
when and how to use, and the pros and cons of each.
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Aside from the above, there are other methods used to grain fill, as for example, using a high
solids polyester primer or two part automotive polyurethane, just to mention a few. These products,
aside from being very expensive, are difficult to use and apply, have a relatively tight window of coat
over or other constraints, thus only very experienced finisher or refinisher would use such products.
Therefore, these methods are left out from this article and discussed in a follow up article. The follow up
article will discuss the use of a conversion varnish (post catalyzed finish) as pore-filler.
For best grain filling results, it is recommended to sand the raw wood to 220 grit, finer if
possible, prior to grain filling. The more you invest in up front sanding the less work you will have to do
later on.
GRAIN FILLING WITH A SANDING SEALER
With small pore or tight grain woods such as birch, maple and cherry, and even on more porous
woods as mahogany and walnut, high-solids, heavy-bodied, stearated sanding sealer can be used
effectively to fill the grain. A heavy-bodied sanding sealer is one that is ready-to-spray as supplied,
packing solids between 24 and 30%, by weight not volume. If the sanding sealer has to be thinned to
spray, or solids are less than 24% when applied, grain filling results will be less than satisfactory due to
excessive shrinkage.
However, this method is only applicable when top coating with a conventional lacquer or a precatatalyzed lacquer that is applicable over a stearated sanding sealer. Most pre-cat lacquers currently on
the market and definitely post-cat lacquers, varnishes (conversion) and polyurethanes (two component)
cannot be applied directly over stearated sanding sealers unless bridge-coated with a non-stearated vinyl
sealer. The “metallic” soap found in most sealers and some vinyl sealers, typically as zinc or aluminum
stearate, designed to make sanding easier, can and will interfere with proper cure of these topcoats. The
acid catalyst built-in or post added to these top coats for cross linking purposes to increase hardness, mar
and chemical resistance, will migrate to and react preferentially with the metal portion of the lubricate
(stearate) in the sealer rather than cross-link within itself. That is, the acid of the catalyst is a stronger
acid than stearic acid, thus the metallic portion from the “metallic soap will debond from the lubricating
soap and bond with the stronger acid of the catalyst. The end result may be a soft finish with inferior
durability properties. The “metallic soap” in sanding sealers, though not a pigment in the classical sense,
can offer decent grain filling properties if applied properly.
When grain filling with a sanding sealer over open grain woods, apply a heavy double-pass coat
with and against the grain. A double-pass coat constitutes first spraying with the grain, waiting 30
seconds or so then spraying against the grain or, in perpendicular direction from the initial coat. It does
not matter whether the initial pass coat is with or against the grain, so long as the two pass coats are
perpendicular to each other and closely as close equal in thickness. The purpose of spraying with and
against the grain is to have the two 90 degree films flow into each other, providing better film
characteristic and filling properties, particularly on porous woods. Allow the double pass coat dry at
least two hours before applying the second double-pass coat. Do not sand the first double-pass coat.
Apply a second double-pass coat in similar manner to the first double-pass coat. Let the second doublepass coat dry overnight - the longer the better. The extended drying time for both double-pass coats
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allows the latent solvents (tail solvents) to evaporate from the four films applied hence, providing better
sanding properties and firm packing of the grain.
After the extended drying time, sand both coats down as far as possible without disturbing the
stain. If filling natural mahogany, for example, sand the sealer coats all the way down to the wood grain.
Use a 220 grit no-load sandpaper. This will pack the grain nicely and yield very good results. Machine
sanding will pack the grain best, but riskier to burn through to the stain, if such exists. Unless you are
filling oak or ash, two double pass coats of a heavy-bodied, high-solids sanding sealer will generally
suffice to give you good grain filling results. On cherry, a single double-pass coat should suffice; two
double-pass coats, however, would yield far better results.
If filling oak or ash, a third double pass coat may be necessary after the aforementioned sanding
then sanded back down to the grain or as far as possible without disturbing the stain. Use 220 grit noload paper or finer for this sanding. Giving this third coat at least two hours drying time prior to sanding
will allow for some shrinkage and make sanding easier and pack the grain more effectively. Letting it
dry overnight prior to sanding will yield even better results because of shrinkage and drop down.
Topcoat as normal with the finish and sheen of choice. It is not necessary to sand between the
first two coats of the topcoat. However, sanding after the second coat is a must to remove imperfections,
which will telegraph as added coats are applied.
Building a finish with sanding sealer rather than leaving its residue in the grain only must be
avoided. First, to make them sand easier sanding sealers are soft by design, which will leave a soft base
to the overall finish thus, susceptible to press marks and inferior mar resistance. Second, have lousy
water, heat and alcohol resistance. Equally important, unless you are looking for that aged look in your
grained filled finish, cracking of the finish within a year could also result. To achieve a crackled finish
multiple coats of a soft sanding sealer is used (without sanding between coats) as an undercoat over an
extremely hard topcoat. Moreover, that currently nicely filled finish will look much less attractive in
about thirty days due to film shrinkage and dropping of the built up sealer.
When pore filling with a sanding sealer under pre-cat or post-cat lacquers or conversion
varnishes, seal the sanded sealer with one to two coats (single pass) of a non-stearated vinyl sealer
without sanding between the coats and after. Vinyl sealers are hard to sand but, more important, are
highly flexible and offer excellent resistance to heat and moisture. Used under non-self-sealing pre-cat
and post-cat finishes, nearly guarantees these high performance topcoats from cracking and chipping. As
such, two coats are recommended and, be left in tact and not sanded. Any added staining or stain repair
due to burn through should be done prior to applying the vinyl sealer coat(s).
GRAIN FILLING WITH A TOP COAT
Grain filling small pore woods such as birch, maple and cherry can also be achieved with highsolids, high-build top coat or a self-sealing, pre-cat lacquer that is ready to spray apply as supplied at
minimum of 28% solids. If using a pre-cat or post cat lacquer to grain fill and finish, it is advisable to
apply a double pass coat of non-stearated vinyl sealer first under most pre-cat and post-cat lacquers to
avoid cracking as wood expands and contracts. Avoid using a catalyzed vinyl sealer so that it can move
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with the wood. Do not sand the vinyl sealer after its application. Let dry thoroughly then apply a double
pass coat of the pre-cat or post cat lacquer wait at least two hours then sand down as far as possible, or at
least smooth.
Over dark woods or dark stain will require many coats, a gloss version of the topcoat should be
used up to the final coat, which will then dictate the final sheen. That is, if dull satin is the desirable final
sheen, build with high gloss or at least gloss then, apply the final coat as dull satin. This will avoid
interference to clarity typically caused by flatting agent in the finish.
For best results apply three double pass coats without sanding between the coats with ample
drying time between each coat. No more than three coats per day and, at least two hours drying time
between each coat. Let the three double pass coats dry overnight, or longer if possible because of
shrinkage, then machine sand the finish down as far as possible without disturbing the stain (if exists)
using 220 no-load sandpaper. As aforementioned, this will pack the grain and fill the pores. .
.
Upon sanding down as far as possible to the wood grain without disturbing the stain, apply
additional two (2) to six (6) double pass coats, lightly scuffing after every other coat. Sand the last to the
final coat smooth prior to applying the final coat(s). Keep in mind that maximum thickness for most
pre-cat and post cat lacquers is 5 to 6 wet mils, hence building up with too many coats could be
prohibitive due to cracking. A very popular pre-cat lacquer currently on the market has a restriction to
four wet mils coats or about 1.2 dry mils. Therefore, such restrictive pre-cat lacquers should not be used
for grain filling.
This method can yield good results on mahogany, rosewood or walnut, but not so on oak, ash or
chestnut unless “hot lacquer” are used to fill the grain and as a top coat. Within 60 days, the finish will
shrink and the pores will show up again. A “hot lacquer” is a high solids (reaching 40% by weight),
thick lacquer whose viscosity is reduced by first passing it through a heat exchanger at about 180
degrees just prior to spraying instead of reducing with a thinner. As such “hot lacquers” tend to shrink
less than conventionally sprayed lacquers or sealers. However, development of high-solids-self-sealingready-to-spray pre cat lacquers and lacquer-based pore-fillers have made “hot lacquers” a finish of the
past.
LACQUER-BASED QUICK-DRYING GRAIN FILLER
On medium pore woods, light grain or light stained larger pore woods, grain filling can be
achieved successfully and rapidly with a lacquer-based wood grain filler. It contains a proprietary
polyolefin based filler dispersed in lacquer rather than pigments, stearates or silicas. It can fill the grain
rapidly, due to its quick drying time. However, clarity can be an issue if too many coats are applied
and/or insufficient drying time is allowed between the coats. For best appearance on mahogany,
rosewood walnut, only two double pass coats should be applied, with at least two hours drying time
between coats. On natural oak, ash or chestnut, up to four (4) coats can be applied successfully with
negligible sacrifice in clarity.
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There are two, yet very significant added advantages to using this type of grain filler over
conventional oil-based filler. It eliminates the need to seal the stain and often bare wood prior to filling,
and the need to wash coat after the grain filling prior to topcoating.
Lacquer tintable stains and colorants can easily be added to this type of pore filler to achieve a
custom color or, where staining and filling is needed in a single step. Non-Grain Raising (NGR) stains
are also suitable as colorants, however, one must be cautioned that these can fade because of the dyes
that are used as their colorant. Water or alcohol type dye stains are generally not compatible with this
type of filler. Any staining should be done prior to filling and re-stained if necessary after filling.
Glazing can be done between filler coats, or after to even out the color or for emphasis.
For best results brush or spray apply this filler. On large surfaces, applying by spray will enhance
productivity and, should yield best results. When brushing, a small amount of lacquer retarder or a flowout solvent may have to be added to avoid brush marks. Brushing should be with the grain and a liberal
coat applied as though brushing a varnish or paint. That is, loading the brush is essential. Figure 1
depicts the proper way of brushing on this filler. When pore filling small areas, you can brush this filler
first against the grain, which will pack the pores, then brush with the grain subsequently to even out the
film.
With spraying, apply a double-pass coat thinned to spray consistency per label directions.
Avoiding or minimizing thinning the filler to minimize shrinkage and dropping. For best spray
application results, apply with a gun having an atomizing set designed for latex or enamel paints, or 2.0
to 2.5 mm.
Allow the first coat to dry for at least two hours. Unlike grain filling with a sanding sealer or a
top coat, sand smooth only; no need to sand off this filler as shown in Figure 2. The idea is to make the
filler smooth yet leaving most of it on the wood’s surface to allow for shrinkage and dropping into the
pores.
It should be mentioned that this lacquer based filler tends to sand easy like a sanding sealer but,
unlike a sanding sealer is not as soft nor sensitive to heat or moisture. Apply subsequent coat(s) as first,
allow amble drying time between coats (the longer the better, within reason), sand smooth not off, then
top coat as normal with a conventional, pre-cat, or post-cat lacquer, varnish or polyurethane. Excellent
and fast grain filling can be achieved via this method.
The use of lacquer based grain fillers is fast and easy, but it does have its limitations. Its overuse
to fill large pores can affect clarity. That is, clarity diminishes as the number of coats increases. Keeping
to two coats and sanding this filler smooth, not off, will offer excellent results.
CONVENTIONAL OIL-BASED PASTE WOOD FILLER
When you need that high gloss or wet look, smooth-as-glass finish with clarity and depth equal
to the waters off Bermuda, your only choice is conventional oil-based paste wood filler. Need that
Steinway Piano look over dark stain on cherry, mahogany, walnut, oak and ash, there is no substitute or
easy way out. However, use of burlap and a lot of elbow grease is no longer a requirement to achieve the
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sought result. The methods given below are faster and more efficient with equal and often better results,
provided the directions provided are followed to the tee.
Conventional oil-based paste wood filler generally contains silex (a finely ground quarts or flint),
dispersed in a mixture of slow drying solvents (like mineral spirits), oils (linseed oil), driers, and varnish
or alkyd resin acting as the binder. Some high-end paste wood fillers contain finely ground glass in place
of silex which yields greater clarity and depth over silex. Because of the type and the particle size of the
pigments used, this filler provides pore filling properties, minimum shrinkage and clarity above all the
aforementioned. Its major drawbacks however, are: time consuming application, slow drying time, and
excessive elbow grease and sanding to remove the excess and achieve a smooth fill. Significant gains in
productivity can be achieved by following application directions given below.
Because of its slow evaporating solvents and oils, it takes significantly longer to dry which, can
be both an advantage and disadvantage. The longer drying time provides ample working time when pore
filling manually large surface areas such as conference room tables. The long drying time can however,
hamper productivity when a piece of furniture has to get to a customer in a hurry.
It is generally offered in a concentrated form and too thick or viscous for use as supplied. It
should be thinned about 25% by volume to a brushable consistency with mineral spirits or paint thinner,
naphtha, turpentine or VM&P Naphtha. Do not thin with linseed oil or any other oil. The thinned
product should have the consistency of brushable house paint or brushable varnish. Thinning with
VM&P Naphtha can cut drying time in half, from 24 to 12 hours. Speeding up drying time can be a
significant boost to productivity, however, VM&P Naphtha is highly flammable and must be used with
extreme caution. When thinning the filler, it should brush easily, yet have enough solids and body to fill
the pores properly with a single application, and sustain negligible shrinkage upon drying and curing.
Avoiding second application gains significant productivity.
Applying the filler over raw wood or a stain directly may change the color of either, and/or,
cause the stain to bleed into the filler. This is because such fillers have elevated pH caused by the
presence of the silex pigment in them. Since all woods in most areas have a moisture content between 6
and 15%, application of the filler can causes muddying or darkening of the grain as a result of its
elevated pH. The increased pH can also draw tannin out of the wood grain and converting it to tannic
acid causing murky or muddy appearance to the stain as well. Therefore, protecting the color of the
natural wood grain or the stain is essential to a discriminating end result.
To protect the color of the wood or stain, it is customary to apply a wash coat of sanding sealer,
vinyl sealer or shellac prior to application of the filler. A typical wash coat of sealer consists of 1 part
sanding sealer thinned with 5 to 10 parts of lacquer thinner or a suitable solvent such as alcohol to
shellac. The wash coats must be thin enough in solids to avoid temporarily filling-in of the pores. This
could make the filler “drop” and/or shrink over time caused by shrinking and/or dropping of the wash
coat or, become pulled from the surface when the excess is scraped or sanded off upon application.
When brushing on the filler, brush in the direction of the grain, applying a liberal coat like
varnish or house paint as shown in Figure 3. The brush should be relatively stiff and short bristled so as
to pack the filler into the pores while brushing. Make certain to mix the container with the filler
frequently during application to prevent settling of the pigments. It can settle out quickly.
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Upon application, let the filler dwell on the surface for a minute or so, but not haze up, then
squeegee off the excess filler across the grain using a wide body plastic putty knife as shown in Figure 4.
Removing the excess filler this way packs the pores well. As such, will minimize or prevent shrinkage
and possibly avoid a need for a second application. Equally important, will reduce the need for excessive
sanding upon drying, a nightmare in itself. This procedure is by far simpler, faster and easier to use than
the old burlap method of removing the excess. If the filler hazes up too soon, it will be difficult to
remove the excess by above procedure because of partial drying. It will necessitate using burlap to
remove the excess. Therefore, avoid filling large areas all at once or speeding up the drying time. Work
with small sections so that the excess filler can be easily removed before it dries via squeegee.
Depending on the thinner used, let the filler dry at least 12 hours prior to sanding and staining,
and at least 24 hours prior to sealing or top coating. The filler must dry and cure thoroughly, especially
prior to sealing or top coating. Sanding too soon will gum up the paper, while the friction from the
sandpaper can cause lifting and/or shrinking due to excessive heat. Sealing or top coating too soon with
a lacquer can cause lifting of the filler, particularly if using a slower drying lacquer, such as a pre-cat.
Commonly available in the most popular colors of natural, walnut and mahogany, it is tintable
with Japan colors and U-T-C colorants and most wiping wood stains. On oak and ash, one can enhance
the beauty of the grain by using contrasting fillers to emphasize the pore pattern. Unless the pore pattern
is to be emphasized, typically the filler is applied in natural color over stained wood where the stain has
been protected with a wash coat from any color change. It is generally difficult to stain over such fillers
since there is no porosity left to accept color however, glazing over a such filler is not a problem.
This extensive drying time can be a major handicap for the finisher or refinisher, but often
unavoidable to achieve a high build, “wet look” finish. In situations where the aforementioned end result
is not required, but the pore must still be filled, there is an easier and faster way to get there. Spray on
the filler instead as described below.
Thin the above conventional oil-based paste filler with VM&P Naphtha at a ratio of 20% filler to
80% naphtha. The ratio will vary depending on the porosity of the grain. That is, on cherry the ratio may
be 10% filler to 90% naphtha (VM&P) while for walnut or mahogany 20/80 could be the best ratio.
Keep in mind that VM&P naphtha is inflammable and must be used with caution. However, if you are
spraying lacquers already, spraying the aforementioned thinned out filler should not be a problem in an
explosion proof environment.
Spray apply this filler with the grain as though spraying a sanding sealer. Stain and/or wash coat
first before applying the filler. A single spray pass will generally suffice unless the filler is extremely
thin in solids requiring another pass against the grain about 30 seconds later or so. Trial and error and
experience will master you best results. Let the filler dry for at least two hours then sand off the excess
without disturbing the stain. Once you master the spray on technique, aggressive sanding should not be
required. Blow or wipe off the sanding dust then feel the surface with your hand. If you can still feel the
grain, spray on another coat of the filler. Allow to dry as aforementioned then, sand the excess off with
the grain. Wash coat with a vinyl sealer then finish as required.
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Finally, a stain of choice can be added to this spray on filler so as to stain and fill in a single step.
Results can be good to excellent depending on experience but, more important, a wash coat step to
protect the stain can be avoided. For best results, it is always better to stain first, then the filler tinted
with the same stain to avoid burn through in sanding.
Table 1
Pros
Cons
Type Of Wood
To Use On
Filler Type
Application
Sanding Sealer
Spray
Easy To Apply
Fast Drying
Easy Sanding
Good Clarity
Soft
High Shrinkage
Requires Excessive
Coats
Top Coat
Spray
Easy To Apply
Quick Drying
Good Clarity
Hard Sanding
Shrinks Over Time
Lacquer-Based
Brush, Spray
Easy To Apply
Quick Drying
Easy Sanding
Good Clarity
Productive
Low Shrinkage
Excessive Coats Can
Affect Clarity
Medium to light color large
grain: mahogany, walnut,
rosewood, oak, ash, chestnut
Conventional
Oil-Based
Brush, Spray
Excellent Clarity
Excellent Depth
Minimum
Shrinkage
Difficult To Apply
Long Drying
Long Curing
Hard Sanding
Anti-Productive
Laborious
Large pore woods, dark
stained mahogany, walnut
rosewood, oak, ash, etc
Shrinkage
(1=least)
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Small pore, tight grain
woods: birch, maple, cherry
Small to medium pore:
birch, maple, walnut, some
mahoganies
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Copyright 2001-2005 by Erick Kasner
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