Rubbing Out A Finish
What makes the difference between a good finish and a
great finish? Rubbing out the finish. From the second you start rubbing
the finish, you start to improve the surface tremendously.
The main objectives are to remove any small dust nibs
that have gotten trapped in the finish before it set up, smooth out any
remaining brush marks or over-spray, remove any other minor imperfections
in the finish and finally achieve the desired level of sheen (from satin
to high gloss). Some very good examples of rubbed finishes are: pianos
that have a high gloss (mirror-like) look, electric guitars, and expensive
conference tables. Rubbing does take time, but the extra time and effort
are surely worth the results.
Many topcoat finish can be rubbed out successfully. Some
will produce much better than others. There are two major qualities that
determine if a finish can be rubbed well; the hardness of the finish after
it has cured and whether multiple coats of a finish fuse together forming
Multiple coats of lacquer and shellac will fuse together
forming one layer. These finishes cure brittle and hard making them easy
to rub. The more brittle a finish after it cures (no matter how hard)
the easier it will scratch and therefore the easier it will be able to
rub using fine abrasives.
Conversion varnish and waterbased finishes also fuse together,
although not as well as shellac and lacquer. These finishes cure tough
and therefore do not scratch as easily as shellac and lacquer and are
more difficult to rub out.
Finally, standard varnishes and polyurethanes also cure
tough and therefore are harder to abrade than other finishes. What really
makes these finishes difficult to rub out is that they do not fuse together.
Each layer sits on top of the previous coat creating multiple layers.
This is the reason you absolutely need to scuff sand between coats of
these finishes, if you do not, each coat will not adhere to the previous
one. When you rub a finish, you are cutting some of it away. If you cut
through the top layer into a previous layer in some spots, you will leave
a visible mark. These marks are called witness marks. These marks usually
appear as irregular spots with a white ring around the perimeter. The
only way to eliminate these marks is to completely rub through the top
layer evenly exposing the layer below. This is very difficult do do, especially
when the thickness of each coat is measured in thousands. If you have
never rubbed out a finish, I suggest you start on one of the easier types
to rub, such as lacquer or shellac.
Today, most finishes applied in cabinet and furniture
shops are rubbed out using buffing machines. These machines or portable
buffers have cloth or foam wheels that are charged with rubbing and polishing
compounds. The process is much the same as when the paint job on the body
of an auto polished, first with a coarser rubbing compound, then with
polishing compound and finally waxed. This method speeds up the rubbing
process. If you have a shop, or can afford a buffing machine, I suggest
you look into this method. Because most of us do not have buffing machines,
I will go over the manual rubbing process which was and still is used
by many master craftspersons.
After the last coat of finish has been applied, you will
need to set the workpiece aside for a time for the finish to cure before
you can rub it out. Depending on the type of finish you have applied,
the temperature, humidity and how many coats of finish you have applied,
the finish can take anywhere from 36 hours to a couple of weeks to cure.
I strongly suggest waiting as long as you can, especially if you are in
a high humidity environment and you have applied more than 3 coats of
1. Wet Sanding
The first step is to remove any dust nibs and smooth and
level the surface. You will need 600 grit silicon carbide wet or dry sandpaper
(black color) and some type of lubricant. Usually water or oil is used.
You can purchase paraffin oil or rubbing oil from woodfinishing supply
companies. Water will make the paper cut quicker, oil will slow down the
cutting. I suggest you start with oil because it will be safer and there
will be less of a chance of removing too much finish. If you cut through
all the coats of finish in some spots, you have to start all over by sanding
and applying more coats, so BE CAREFUL, especially on edges and corners
where it can be very easy to cut through to the raw wood.
Apply a thin layer of oil to the surface of your finish.
I usually pour a little on the palm of my hand and wipe it on the surface
evenly. Next, take a 1/4 sheet of 600 grit wet or dry paper and fold it
into three, keeping the abrasive sides outside. Now gently start sanding
the surface taking long, straight strokes with the grain. Once you get
to the trailing end of the surface, lift the paper right before the edge.
The motion is that of a plane taking off a runway. This will prevent you
from removing too much finish at the edge. After making one stroke, come
back to the leading edge and start another pass, slightly overlapping
the first. Continue this method until you have sanded the entire surface.
Periodically, you will need to wipe off the mix of oil
and dust to check your progress. You will want to obtain a uniform sheen.
Shiny spots are low areas that the paper has not touched yet. Apply more
oil and continue to sand. You will need to sand more in order to level
the finish enough to the point where the shiny spots are gone and the
whole surface has a uniform sheen. Once this is accomplished, you should
have a beautiful smooth, satin sheen. If you are happy with a satin sheen,
stop here. All you will need to do is clean the surface with a rag slightly
dampened with some mineral spirits and then apply a coat a paste wax or
liquid polish if you wish. However, if you wish to obtain a higher sheen,
you will need to continue the rubbing process using a finer abrasive such
as pumice powder, which is covered in the next step.
2. Rubbing with Pumice Powder.
From here on, the only reason to continue to rub is to
bring up a higher sheen or gloss. Wet sanding removed the dust nibs, leveled
the surface and produced a satin finish. In order to produce a higher
sheen or gloss, we need to use a finer abrasive. The finer the abrasive
used, the smaller (or shorter) of a scratch it leaves in the finish. Although
the scratches left by wet sanding with 600 grit paper are much too small
to be seen with the naked eye, these scratches are still to large to produce
a high sheen. Pumice powder is a very finely ground volcanic rock. Available
in various grades. Usually the finer grades like FFF and FFFF are used
in the rubbing process. I suggest FFFF pumice which is the finest of the
grades. Pumice powder will will make the scratches left by the 600 grit
paper smaller. The smaller the scratch, the more the light will reflect
off the surface. The larger the scratch, the more the light gets trapped
in the scratches producing a lower sheen.
For this operation you will need two felt blocks approx.
1/4” to 1/2” thick and 2” wide x 4” long, FFFF Pumice powder and more
of the rubbing oil you used in the wet sanding operation. The felt block
and pumice can also be purchased from woodworking or woodfinishing supply
Apply some rubbing oil to the surface, and sprinkle a
little pumice evenly over the oil. Next, using one of the felt blocks,
start to rub gently with the grain using the same motion described for
the wet sanding operation. Just like the wet sanding step, periodically
wipe off the surface and inspect it for a uniform sheen. Re-apply the
oil and sprinkle a little more pumice over the oil and continue. Once
you have achieved a uniform semi- gloss sheen, you are done. Once again,
wipe the surface clean and if you wish you can apply a coat of paste wax
or liquid polish.
At this point, you have probably guessed that there needs
to be one more step if you wish to obtain a high gloss finish. Yeah, you
2. Rubbing with Rottenstone.
Rottenstone is also a very finely ground rock. It is even
finer than pumicestone. Using rottenstone will produce the finest mirror-like
finish. Rottenstone only is available in one grade and can also be purchased
at woodworking and woodfinishing stores. The process is exactly the same
as the pumice process, just make sure you do not use the same felt block
that you used for rubbing the pumice, if you do, the pumice will mix with
the rottenstone and give you an uneven scratch pattern. Once done, again,
clean and apply paste wax or liquid polish.
The wet sanding, pumice, rottenstone is not the only method
for rubbing out a finish. As I mentioned earlier, one may use rubbing
and polishing compounds along with a buffing machine. There are also superfine
abrasive papers that can be used in place of the above methods. One of
these is called Micro Mesh which is a series of abrasive snadpapers that
have a rubber and cloth backing. These papers start at 1500 grit and go
up to 12,000 grit, leaving a scratch pattern that is so uniform and small
that it produces a super mirror gloss finish. I plan to cover the use
of this paper in a future article.