Rubbing Out Wood Finishes
Copyright ®2000 by Jeff Jewitt. All rights reserved.
Not reproducible in any form, written or electronic, without permission.
Note: Abrasive grits
referenced in this article are P (FEPA) graded. If you are using a
CAMI grade abrasive, use the chart
here to convert.
Finishes
rarely look or "feel" right if left right off the brush or spray gun.
Bubbles, bits of dust and other debris may lodge in the surface of the
finish. You can feel these with your hand as you pass it over the surface.
Brush marks and
patterns from spray application will leave an irregular surface which
is noticeable, particularly on gloss finishes. Rubbing out a finish is the
last step in finishing and the object is to remove imperfections, even out
and smooth the surface, and establish a consistent sheen to the finish.
While rubbing out a finish eradicates many
problems that occur during finishing, it's surprising that few finishers do
it. No doubt there is a lot of fear generated by the concept of sanding and
rubbing a clear finish that's only six-thousandths of an inch thick. But
rubbing out is a lot easier if you understand the theory of what you're
doing and you do it in stages. This article will show you how to rub out any
clear finish using both traditional and modern techniques. You'll be able to
control the degree of luster (flat, satin or gloss) by the materials and
techniques that you use.
Which
Finishes to Rub
Any film-forming finish can be rubbed out, it's
just that some finishes are easier than others. Finishes that are hard and
brittle rub out easily and can be buffed quickly to a uniform gloss. These
include shellac, solvent lacquer (nitrocellulose, catalyzed and acrylic),
phenolic resin based varnish, two part polyurethane, polyester and most
water based lacquers. Finishes that are soft, or tough and flexible can be
rubbed out, but they don't always take a uniform sheen and are difficult to
polish to gloss. These include oil finishes, long oil varnishes, oil-based
polyurethane and catalyzed varnish. Any gloss finish can be rubbed
out to flat or satin, but the real difference between a brittle finish and a
flexible finish is when you try to polish it back up to gloss. Elastic
finishes tear when you abrade them. It is impossible to establish a
well-defined, even pattern of scratches. Hard, brittle finishes leave a well
defined scratch pattern and by making the pattern of scratches smaller with
each abrasive change, the finish will eventually polish to gloss.
It's like the difference between polishing the sole of your shoe compared to
polishing a piece of brass.
Satin, semi-gloss, flat and dull finishes are
also not suitable for rubbing, although they can be abraded and then rubbed
to remove imperfections. These finishes produce the various sheens --
satin, flat, etc., by the addition of fine silica which cause diffusion
(scattering) of light at the surface of the dried finish. Abrading and then
rubbing these
finishes destroys this effect and may not product adequate results.
Materials
The materials needed for rubbing out are
abrasives. The first two stages, removing imperfections and flattening the
finish, are done with abrasive papers. The last stage, polishing to the
desired sheen, is done with steel wool, synthetic steel wool and abrasive
powders. These can be mixed with a carrier like water or oil by the user or
are supplied in paste form, which is a mix of solvent and waxes. Some
materials have been around a long time and others are modern versions.
Abrasive papers - The traditional
paper used for removing imperfections and leveling the finish are the
silicon carbide wet/dry papers. They should be used with a lubricant.
Rubbing oil, mineral spirits or Naphtha are used by many shops but some prefer
to use soapy water with a dash of dish-washing soap added. This reduces the
surface tension of the water and allows it to "wet" the finish surface
better.
Soapy water generally is less messy than using oil and a little easier
on the hands.
For dry sanding, non-loading aluminum oxide or
silicon carbide papers are used. They will clog quickly with finishes like
oil based varnish, solvent lacquer, some water-based products, lacquers and
shellac. On other finishes like acrylic (water and solvent based)
polyurethane (water and solvent) and catalyzed varnishes and lacquer they
don't clog as quickly. The advantage of dry-sanding is that you have better
control because you can see what you're doing. Wet-sanding with a lubricant
creates a slurry which gives a false illusion of a thick finish. It's easy
to sand through to sealer or color coats with wet-sanding.
Several newer papers are available which are worth
trying. They use new, proprietary processes to ensure precise, uniform grit
size. Traditional processes allow as much as 35% deviancy from rated grit
size. Mirka
Royal
series and
3M
Fre-Cut Gold
are two products which utilize uniform grit size. This makes them
costlier, but they cut much faster and more efficiently. Both of these
papers are "P" graded which is a different grading that you may be used to.
In instances where
I used to use a 400 grit standard (CAMI) paper, I use 600 grit or
higher with the new papers.
Steel-wool, Synthetic steel wool -
Steel wool is strands of steel spun into a pad. It is graded in the aught
system from 3 (Very Coarse) to 0000 (Finest). The only grade I use is 0000.
Synthetic steel wool is a non-woven
synthetic fiber pad that is impregnated with abrasive particles- usually
aluminum oxide. Synthetic steel wool is graded by generic description - Coarse to
Super Fine. Both of these act like abrasive papers, but because they are
cushioned abrasive, they do not shear off high spots like dust pimples or even out
irregularities in the finish surface. They ride over high spots. They are generally used after wet-sanding to establish a consistent scratch
pattern for flat and satin finishes and with a lubricant - either
soapy water, thinned wax or a lubricant made specifically for steel wool,
called wool-lube. The gray color pad
is equivalent to 000 steel wool and the white pad is impregnated with talc
so it is used for polishing.
Abrasive powders -
Abrasive powders
are used for polishing up to the desired gloss after sanding and steel-wooling.
Traditional powders for polishing are pumice (powdered volcanic glass) and
rottenstone (powdered decomposed limestone).
These mineral powders are composed of sharp, angular
fragments that scratch the finish. Pumice is sold in different grades from
1F (Coarse) to 4F (Fine). Rottenstone is finer than pumice and is sold in
only one grade which is fine enough to polish a surface to gloss.
The
abrasive powders above can be combined with other materials to produce
liquid suspensions or pastes which can be used by hand or machine. You can
find these products at automotive supply shops. Two manufacturers, 3M and
Meguiars, have systems of rubbing compounds that work very well on furniture
finishes. However, we've found the
Menzerna line of polishes we sell to be
superior. If you use these products stay with one manufacturer - since a
final rubbing compound from one manufacturer may be different in abrasive
grit than another.
Stages in Rubbing Out
The process of rubbing out is divided into
stages - removing imperfections, leveling then polishing. The first two
stages- removing imperfections and leveling involve abrading the finish, the
idea being that finish is removed to establish a smooth, level surface which
is then polished. Polishing is bringing the finish up to the desired luster
by creating a pattern of scratches. The difference between the two is cloudy
because both methods involve removal of finish. The difference is in the
materials used. Abrading involves abrasives up to 600-1200 grit. Polishing
uses abrasive materials higher than 1200 grit. Because the scratches left by
this grit and higher are so small, finish removal is minimal. Before you
start rubbing away, there are several important points to consider.
If the finish is gummy and loads up the paper in
the initial leveling, it's not dry enough. Let it cure longer.
Rubbing Out By Hand
Using Traditional Materials
Rubbing out a finish using traditional materials
like wet/dry papers, pumice and rottenstone is straightforward and easy to
do without an investment in materials. The same procedure can be used with
modern abrasives, but the general idea is the same.
Removing imperfections - Start
with the lowest grit wet/dry paper you can to remove the imperfections
efficiently. I usually start with 400 or 600 grit but will go to 320 if the surface
has big hunks of debris. Wrap the paper around a backing block like cork and
sand just enough to remove the tops of pimples so that they're level with
the rest of the finish. Soapy water can be used on most finishes but can
cause problems with shellac and some water-based finishes. finishes. Use
stearated aluminum oxide and dry-sand the finishes with a backing block. On
textured surfaces with bad dust pimples, use your hand as a backing block
and go easy. At this point the
surface should have alternating dull and shiny spots when viewed in
backlighting.
Leveling - Once the high spots,
drips and other defects are removed, switch to the leveling step. The goal
here is to establish a consistent scratch pattern across the entire surface
of the wood. Try to do this step with 600 grit if you can. It makes the rest
of the procedure go quicker. If the surface has rough brush marks or orange
peel, you'll probably have to start at 400 or even 320. Either way, take
some wet/dry paper and tear it into quarters. Wrap a quartered sheet around
a cork block and squirt some soapy water onto the surface. I use a plant
mister. Do the edges first. Work the edges all the way around the perimeter
of the top with the paper with short, choppy strokes. Then work the
center, working in one direction only (typically with the grain) Brush
aside the slurry and look at the top in backlighting to check your progress.
What you're aiming for is a surface with no shiny spots. Check the surface
of the paper often- if it starts to show clogging, shift the paper to expose
fresh grit or change the paper. Don't try to economize by using clogged
paper, you'll create problems. If you set the paper down for any reason, set
it on a clean surface. If it picks up any debris- it will scratch the fresh
finish.
Brush aside the slurry and wipe the piece with
naphtha if you want to accelerate drying of the water. When viewed in
backlight, the entire surface should show a dull scratch
pattern. More than likely you'll see areas that are still shiny.
Squirt some more water on the surface and work these areas until but don't
overdo it- you'll create a visible hollow. Work the shiny area a little then
feather it in to the rest of the surface. Work slowly and deliberately until
the entire surface is dull. If there are several small areas that are shiny
- like partially filled pores, don't try to sand them level. They won't be
too visible when the entire surface is buffed. If there are shiny spots
around the edges and close to the edge, take some 0000 steel wool and using
it dry, rub the surface until it's dull like the rest of the surface.
When you're satisfied with leveling, switch to
the next grit wet/dry paper and do the same as above. Pay attention to the
edges first then switch to the center of the board. Continue on to 800 grit.
Now you have a choice. If you want a satin
finish take a pad of 0000 steel-wool, unravel it and fold it back into
quarters. Wrap it around a cork block or soft piece of wood or plywood with
cork glued to the bottom. Squirt some soapy water on the board and then
apply some steel-wool rubbing lubricant (Behlen
Wool-Lube) on the pad or the board. Rubbing with the grain, make
3 or 4 complete passes over the surface, slightly overlapping each pass with
the next. Then switch to a fresh area of the steel wool and repeat. Do this
2 more times, switching to a fresh area of the steel wool for a total of
12-16 passes. Brush aside the slurry and check to see that you're putting
down a uniform scratch pattern. You may have to let the board dry to see if
you've got it right. If you want a waxed feel to the surface, let the
wool-lube slurry dry, then buff it off, just like wax. The surface will have
a silky feel to it and when viewed in backlight, should look like brushed
metal.
If you
want gloss, skip the above step and continue wet-sanding up to at least 1200
grit. If you want to avoid a lot of work with the polishing compounds, take
it to 1500 or 2000.
Now take some 4F pumice and sprinkle some
on the surface of the wood. Squirt some water or rubbing oil over the
pumice. Wad up a clean dry cotton cloth and working in whatever direction
you want, polish every square inch of the board. Apply a good deal of
pressure and replenish the pumice and water as it gets dry. Let the slurry
haze over, then wipe it all off with a damp rag. Switch to rottenstone and
do the same until the finish is as glossy as you want.
Rubbing to satin by hand is not that hard, but
rubbing to gloss by hand is a lot of work. You can save a lot of time by
investing in a power buffer if you rub out large surfaces. Power buffers are
only effective on gloss finishes. Satin finishes still have to be
hand-rubbed.
Rubbing Out Using Modern Materials
The basic concept of rubbing out a finish using
modern materials is exactly the same as rubbing out using traditional
materials. I always prefer to wet sand by hand, but if you want, switch to
air-powered equipment for wet-sanding. The best machine for wet-sanding has
opposing, in-line pads which do not make a circular scratch pattern. The two
big manufacturers of these are Stuhr and National-Detroit.
I've used an orbital pad sander with good success, but move it very slowly.
You can dry sand the finish level with an electric random-orbit, but only
certain finishes. Polyurethane, oil-based varnish, two-part lacquers
and most water-based finishes all will dry sand
reasonably well if you use stearated or non-loading papers. Obviously, do
not use electric sanders when wet-sanding.
Start the process exactly like above with 400 or
600 grit paper. You can use 320 if the surface is badly orange-peeled, but
only if you're sure you have a thick enough finish. Work the edges first,
then switch to the hatching sequence and work up to 600 grit. If you want
satin, you still have to do it by hand, using steel wool and wool-lube (You
can substitute thinned wax if you want).
If you're aiming for gloss, work to at least
1500 grit. Working to a higher grit will make the polishing go quicker.
Move the furniture to an area where flying compound won't be a problem and
put an apron on to cover your clothes. Take some Menzerna 2L (or equivalent
compounding paste) and smear some on the surface or squirt some stripes down
the center of the surface about 8 inches apart. If you have an old pad on
the buffer, spin off the old compound by holding a stick against it while
it's spinning. I like to use 3M buffing pads, but Meguiar's makes foam pads
which work just as well. With the buffer off, smear the compound all over
the surface of the finish. With the buffer held off the surface, turn it on
and holding it a very slight angle (about 2-3 degrees) place it on the
surface of the finish. Move the buffer across the surface of the finish
slowly, trying not to stay in one area too long. I like to work this
sequence just like the wet sanding, I do the edges first, then work the
buffer into towards the center. (You can spritz the Menzerna 2L compound
periodically with soapy water to keep it working)
There is a natural fear to using a buffer on a
clear finish. Try not to be too jerky and work the buffer in smooth,
confident strokes. When working edges, pay attention to the angle and
rotation of the buffer, sharp edges of the top may catch the edge of the pad
and cause kick-back of the buffer. On large objects, like dining room
tables, you can work the top while it's attached to the aprons and legs. On
small, light items like
nightstands you may need to remove the top and secure it with clamps or a
vacuum hold-down.
The scratches from sanding disappear as you buff
and it's easy to see when you're done with the compound. Overhead lighting
or backlighting will highlight errant scratches and you can work any missed
areas with compound. Let the compound haze, then wipe it off with a soft
cloth and examine again. It's important to remove all scratches with the
initial compound. If you don't and you discover scratches with the next
compound, you'll have to go back and work up to the compound you're using.
After using the 2L (or equivalent), I switch to
16 Polishing Paste and repeat. You should see a deep gloss appear at this
point. Cover the entire surface, let the compound dry, and wipe it off with
a soft cloth.
Most finishers apply a glaze or swirl remover at
this point for the ultimate gloss. I switch to
PO91E Intensive Polish and apply the polish
by hand or with the buffer. If you notice swirl marks from the buffing pad,
you can use a swirl-remover, but I find that a light buffing with a clean
dry pad on slow speed works just as well.
Note -- If you
don't have a power buffer, you can always apply and work the polishing
compounds by hand with a soft cloth. Or, if you have a random-orbit palm
sander, purchase a foam pad that's sold as an accessory to fit on your
Velcro sander bottom.
Menzerna Polishes will not create a whitish,
hazy look on water-based lacquers. With other polishes this is usually due
to the presence of aromatics in the mineral spirits which softens the
lacquer, making it hard to polish. If this happens with a polish you're
using switch to the Menzerna. I've never had a problem with these products.
Rubbing Out Difficult
Surfaces
Moldings - Wrap some 800 grit
wet/dry around rubber tadpole sanding blocks to approximate the convex and
concave curves of the molding. After drying, rub the molding with 0000
steel-wool and wax. When dry, the wax can be buffed up to approximate the
sheen of the rest of the surface.
Turned legs - Use 0000 steel wool
and wax thinned with mineral spirits. Avoid hard rubbing - you'll cut
through the finish on sharp details.
Carvings - Carvings should not
have a lot of finished applied in the first place- it destroys detail. When
rubbing use steel-wool and wax and go lightly. Use dark colored wax on dark
woods.
Fixing Mistakes
When rubbing out, you will sometimes rub through
the finish and into bare wood. There are easy remedies for this. If it
happens during buffing I usually continue until the gloss is what I want and
avoid working the area around the rub-through with the buffer. When you're
done, take some alcohol or water dye and color the wood to the original
stain color. Water dye works better, since it won't "take" on the finished
areas. Then take some poster board and drop it down over a running table saw
to make a slit. Tape the board so that the rub-through shows through the
slit and spray some gloss finish (3-4 coats) with a touch-up spray gun or
aerosol can. Don't spray too much, it will create a sharp line against the
rest of the finish. When it's dried after a day, use some 1500 grit sandpaper
(I use the Mirka Royal and dry sand) then Menzerna Polishing compounds and
feather it into the rest of the finish.
If rub-throughs happen during wet-sanding,
you'll have to correct them before continuing. Stain and finish like in the
above procedure, then very lightly dry rub the area with 0000 steel-wool.
This should blend in with the satin finish. If you're going for gloss, you
may want to use several more lacquer application before buffing.
How Sheen is Created
Scratches can produce a flat, satin, semi-gloss,
or gloss sheen by how they reflect light. On a perfect gloss surface, light
is reflected to your eye at the same angle that it strikes the surface. In
technical terms this means that gloss is created when the angle of incidence
is equal to the angle of reflectance. When scratches are put on the
surface and they are larger than the wavelength of visible light- they will
diffuse light away from the eye. Scratches left by 400 grit result in a very
dull surface. From 400-1000 a flat to satin surface is the result. Above 1500 a semi-gloss
surface results. When the width of the scratch is smaller than the frequency
of visible light (as in fine polishing) the surface starts to appear glossy.
Interestingly, if
you look at a satin surface from a low angle (like a dining table top from a
kneeling position) it will start to appear glossy. That's because you're
seeing less diffused light.
