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Creating a Smooth Finish
There are many factors, including thorough
wood preparation to remove machine marks, swirls, scratches & dents,
etc. But when it comes down to it, the one factor that separates a
great finish from an average finish is smoothness.
There are two significantly different types of
finish: penetrating and film-building. A penetrating finish is one
that doesn't harden, so all the excess has to be wiped off after
each coat. Oil finishes are penetrating finishes. Oil finishes
include boiled linseed oil, tung oil, and a mixture of varnish and
one or both of these oils.
All finishes that harden are film-building
finishes. They can be built to a greater thickness on the wood by
leaving each coat wet on the surface to dry. The procedure for
rubbing is different for oil and film-building finishes; I'll
You can create a fairly smooth oil finish by
sanding between coats using very fine grit sandpaper (#320 grit or
finer). Be sure to allow each coat to fully cure, which means
leaving overnight in a warm room. Some oil finishes, such as Watco
Danish Oil and Deft Danish Oil, instruct to apply coats within an
hour or two; following these directions won't produce good results.
You can create an ultimately smooth oil finish
by sanding each coat while it's still wet on the surface using very
fine grit sandpaper. Then wipe off the excess and allow what's left
to cure overnight.
Here's the procedure.
Sand the wood to remove machine marks and
Wipe or brush on a wet coat of oil and keep the surface wet for
several minutes, rewetting any areas that become dull because the
finish has soaked in.
Wipe off all the excess. Be sure to hang your wet rags to dry, or
drape them singly over the edge of a trash can, so they can't
Allow the finish to dry overnight in a warm room.
Wipe or brush on a second coat of oil and sand the surface while
it's still wet in the direction of the grain using #600-grit wet/dry
sandpaper. Sand over all areas with three or four back-and-forth
strokes. There's no gain sanding more than this.
European standard P-grade sandpaper is rapidly
replacing the American standard. Above #220 grit, P-grade numbers
move up much faster than non-P-grade. Sandpaper of #600 grit is
approximately equivalent to P1200 grit; #400 grit is about P800.
Wipe off the excess oil and allow the surface to dry overnight.
Apply a third coat of oil and again sand wet.
Remove the excess and allow overnight drying. This is usually all
you need to do to achieve an ultimately smooth finish, but you can
repeat the procedure with a fourth coat, and with as many additional
coats as you want.
One caveat: Sanding an oil finish wet (or even sanding dry between
coats) is risky if you have stained the wood. You might sand through
some of the color, especially at edges. Sand lightly and carefully.
Film-building finishes include varnish,
lacquer, shellac, water-based finish and two-part catalyzed
finishes. Both varnish and waterbased finish have a version called
polyurethane. This is the regular finish (alkyd or acrylic) with
some polyurethane resin added.
Catalyzed finish is also available in one part
called pre-catalyzed lacquer. Except for varnish, each of these
finishes hardens within a couple of hours in a warm room so several
coats can be applied in a day. Varnish, on the other hand, requires
overnight drying between coats.
The Sealer Coat:
The first coat you apply of any of these
finishes is called the sealer coat. It stops up the pores and seals
the wood. It also leaves the wood feeling rough, so you should
always sand the sealer coat smooth. (Though you could skip the
sanding and still achieve smoothness at the end by sanding just the
last coat, it's easier to sand the sealer coat because it's thin.)
Varnish (not including polyurethane varnish)
and lacquer are more difficult to sand than other finishes because
they tend to gum up the sandpaper. So manufacturers provide a
special product called sanding sealer to use as a first coat under
these finishes. Sanding sealer is varnish or lacquer with a
soap-like lubricant included. Sanding sealer powders when sanded.
If you are finishing a large project such as a
set of cabinets with varnish or lacquer, it will be worthwhile to
use a sanding sealer for your first coat. But if your project is
small, requiring little sanding, it's better to avoid using sanding
sealer because it weakens the overall protection of the finish. The
included soap weakens the moisture barrier and makes this layer
softer than the finish itself.
Instead of using sanding sealer to gain easy
sanding, you can thin the finish itself about half with the
appropriate thinner (mineral spirits for varnish or lacquer thinner
for lacquer). The thinner layer of finish hardens faster so it is
easier to sand sooner.
If you are finishing a wood with resinous knots (such as pine), or
you are refinishing wood with silicone contamination (it causes the
finish to roll up in ridges) or animal-urine or smoke odors, use
shellac as the sealer coat. Shellac blocks off these problems (but
it is not easier to sand). There's no reason to use shellac
No matter what you use for the sealer coat,
sand it after it dries using a grit sandpaper that creates
smoothness efficiently without causing larger-than-necessary
scratches- most often a grit between #220 and #400.
Sanding Between Coats:
It's always best to sand lightly between every
coat of finish to remove dust nibs. This is done easily using very
fine-grit sandpaper: #320 or #400 grit (P400 or P800). Using a
stearated or dry-lubricated sandpaper is best because it clogs
least. This sandpaper has the same soap like ingredient as sanding
sealer and is usually available at auto-body supply stores.
Sand just enough so you can no longer feel the dust nibs. There's no
reason to sand out brush marks or orange peel (caused by spraying)
at this point.
Rubbing the Finish:
When you have applied all the coats you want,
usually three or more including the sealer coat, it's time to make
the surface feel smooth.
If the dust nibs aren't bad, you can usually
improve the feel significantly by simply rubbing lightly with a
folded brown-paper bag. As long as you have allowed the finish to
harden well (so you can no longer smell any odor when you press your
nose against it), the bag will level the nibs without damaging or
changing the sheen of the finish.
To create a more perfect and attractive
surface, rub it with #0000 steel wool or gray Scotch-Brite. Rub in
the direction of the grain. Rub the three or four inches nearest the
ends using short strokes (so you are less likely to rub over the
edges and cut through). Then rub the entire length, being careful to
stop just short of the edges.
You can achieve even better results by using a
soap-and-water or mineral-oil lubricant with the steel wool or
Oil causes the abrasive pad to scratch the
least, but cutting will be slower and the gloss attained higher. You
can try one and then the other on the same surface to see which you
like best. Most professionals use soap and water.
Be careful using any lubricant because if you
cut through you won't see the damage until the lubricant dries.
Rubbing with an abrasive pad, with or without a lubricant, improves
the feel and appearance, but it doesn't remove the flaws; it just
rounds them over and disguises them with fine scratches. To achieve
the ultimate rubbed finish you have to level the finish first and
then rub it.
Leveling and Rubbing:
Leveling a finish is a mechanical exercise
employing the same exact procedure as sanding wood, with two
differences: You use finer grits of sandpaper and you use a
lubricant with the sandpaper to prevent clogging. Here is the
Using a flat sanding block to back your
sandpaper, sand the surface until it is perfectly flat. Use a grit
sandpaper that cuts through the flaws efficiently without creating
larger-than-necessary scratches that then have to be sanded out,
usually a grit between #320 and #600.
Use wet/dry sandpaper (black in color) and a
lubricant of mineral oil, mineral spirits or a mixture of the two.
The oilier the lubricant, the slower the cutting and the less likely
the sandpaper will clog. (I find that sandpaper clogs quickly with a
water or soap-and-water lubricant, but you can use one of these
On unfilled, open-pored woods you may need to
apply more than three coats so you don't sand through. Because
finishes differ in solids content and thus build, and because
everyone applies finishes differently, you will need to experiment
on scrap wood to learn the number of coats necessary so you don't
sand through. Think in terms of four to seven.
A finish has no grain so you don't need to sand
with the grain. In fact, you can sand in circles, which I find
easier, and you can sand cross-grain near the ends (to keep from
sanding through the finish at the edges).
Each time you advance to a finer grit
sandpaper, change directions (circles, with the grain, across the
grain) until you reach your finest grit which should go with the
grain. By removing the sanding sludge with naphtha or mineral
spirits, you will be able to see clearly when you have removed all
the scratches from each previous grit sandpaper (a big advantage
over sanding wood).
You will see your progress better if you use gloss finish rather
than satin. After sanding a little, scrape off the sludge from parts
of the surface using a plastic spreader. If you see shiny troughs or
spots, the surface isn't level. When the surface is an even satin
sheen overall, it is level and you can move to a higher grit
sandpaper to remove the coarser scratches.
Once the surface is level, sand or rub it with finer and finer grit
abrasives until you achieve the sheen you want. Begin by sanding up
to at least #1000 grit, continuing to back your sandpaper with a
flat block or a felt or sponge pad. Then rub with #0000 steel wool,
or with pumice and a mineral-oil lubricant using a felt or cloth
If you want a higher gloss, sand up to #2000
grit (P2000 or higher) and then rub with rottenstone and a
mineral-oil lubricant using a felt or cloth pad. Or use any other
abrasive rubbing compound.