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Understanding Wood Finishes
When selecting the perfect wood finish it’s
easy to become overwhelmed with all of the slick labels and marketing
hype. A quick trip to the local home project center will reveal the
hundreds of different choices. The term finish is often used loosely
to refer to any chemical that is applied to wood. The term actually
refers to a clear protective coating that sits on or in the surface of
the wood.
Stain, Paint, and Finish are the three
primary classifications for common wood treatments. Stains
contain pigments and are used to tint the wood. Paints contain
colored pigments and sit on the wood to form a protective coating.
Some finishes are simply paint with out the pigment that lay
down a clear protective coating. There are five common types of
finishes on the market. They are:
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Oil
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Varnish
and Polyurethane
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Shellac
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Lacquer
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Water-based
Finishes
Applying finishes is one part of woodworking that
doesn’t require many tools. In fact there are only three main tools
used to apply all of the finishes listed above; rags, brushes, and a
spray gun. Many professionals will use a spray gun for smooth even
coats.
When you are selecting a clear finish it is
important to remember the qualities you require from the finish.
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It
must protect the wood.
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It
must be durable enough for the intended application.
-
It
should be as easy to apply as possible.
To protect your project for the long term you
should select a finish that has a maximum resistance to moisture vapor
exchange. Thicker finishes tend to slow down this exchange more. Keep
in mind though that thicker might not always be better. Polyurethane
is more prone to cracking after 4 or 5 coats. In this case, varnish
might be the best choice.
The durability of a wood finish is an important
part of protecting the wood beneath. A durable finish is more
important for a tabletop than a mantle or picture frame. Durability
has more to do with the chemistry of the finish than the number of
coats. For example a single coat of polyurethane is more durable than
multiple coats of a water based finish.
It’ll probably surprise you to learn that the
ease of applications is one of the key factors to achieving a
professional finish. Slow drying finishes like polyurethane and
varnish are relatively easy to apply with a brush. However, their slow
dry time leaves them vulnerable to dust landing on the surface and
leaving spots. Faster drying finishes can be difficult or impossible
to apply without a spray gun. Oil finishes aren’t affected by dust
since they soak into the wood.
Now that we’ve covered some of the basics
let’s take a look at the different type of finishes. There are five
primary types of finishes, a bunch of sub-categories, and a limitless
number of brand names for these finishes.
These types of finishes have been used for
centuries to treat and preserve wood. Oils are different from most
other finishes on the market because they seep into the wood and
penetrate the wood’s fibers. Because of this, oil finishes cannot be
built up to a thick coat like polyurethane or varnish can. They offer
less protection but are also easier to apply which is their primary
advantage. Another advantage is that simply simply wiping on more oil
can often repair minor scratches.
There are two types of oils, those that cure and
those that don’t cure. Oils that don’t cure should generally be
avoided because that can continue to seep into the wood leaving the
surface unprotected. They can also leave a sticky surface on the wood.
Linseed oil and Tung Oil are both oils that cure and work well as
finishes.
This oil is made from the seeds of the flax
plant. Look for “Boiled” linseed oil. This product has an added
metallic drying agent that helps the finish dry in a day. Linseed oil
without this additive can take over a week to dry.
Pure Tung oil is pressed from the nuts of the
Tung tree. Unlike Linseed Oil, this finish does not require drying
additives and cures in several days.
The primary advantage of using these finishes is
their ease of application. Simply wipe it onto the surface with a
clean cotton rag, wait 10 minutes, and applying more oil and allow
time to cure. Always sand lightly between coats. Linseed Oil generally
requires three coats to achieve the “hand rubbed” look. Tung Oil
may require 5-6 coats to achieve the same results. The first few coats
will tend to cure rough but will even out during the last few coats.
Varnishes are one of the most protective finishes
available. This level of protection increases with additional layers.
The primary down side of varnishes is that they are slow drying which
can allow dust and dirt time to settle and damage the finish. Because
of this it is best to finish your project in a clean dust-free room if
possible.
Varnishes are produced by cooking an oil and
mixing it with a resin such as synthetic alkyds, phenolics, and
polyurethanes. Interestingly, polyurethane finish is actually varnish
made with polyurethane resin to make the finish more protective and
durable.
When more oil is used than resin the finish
becomes more flexible. This type of varnish is called “Spar
Varnish” and is ideal for outdoor use because the flexibility makes
it more forgiving of seasonal wood movement.
Achieving a perfect varnish finish is a
combination of skill and experiences as well as access to the right
tools for the job. Work in a clean dust-free environment. The room
should be prepared by wiping it down surfaces with a damp cloth. Also
be aware of dust that can be generated by human skin and clothing. The
wood should be wiped with a tack cloth and the finish transferred into
a separate working container.
Varnish is usually applied with a brush using
long steady brush strokes. In contrast to paint varnish brush strokes
are much more apparent. Work in the direction of the grain and
“tip-off” your brush strokes with lightly brushed vertical
strokes. Varnish should be wet sanded between coats with a fine
sandpaper.
“Wiping Varnish” that has been thinned with
Paint Thinner is also available. Use caution since some manufacturers
refer to their wiping varnish as “Tung Oil”. Read the labels
carefully since these products are completely different. One way to
test this product is to pour some of the finish onto a piece of glass
and allow it to dry. If the finish cures to a hard finish it’s
varnish. This product can be applied with a brush like varnish or with
a cotton rag like an oil finish. The advantage of wiping varnish over
an oil finish is that you can leave some of the finish on the surface
and build up to a thicker coat.
Just to make thing a little more confusing the
finish manufacturers introduced a blend of varnish and tung or linseed
oil. These finishes are applied and act similar to an oil finish but
the varnish adds some additional protection. They will still cure in
the wood like an oil finish and should not be used if you plan on
building up layers on the surface.
Shellac is one of the few natural resins still in
use today. Although not as durable as a Lacquer or Varnish, shellac
still provides a modest amount of protection. Shellac is not the best
finish for tabletops, chairs, and kitchen cabinets due to high wear
requirements of these items.
The big advantage, and the reason the finish is
still in use today, is that Shellac is alcohol based which makes it
very fast drying. Because Shellac is fast drying it is less likely to
collect dust like varnish. It is often too thick to apply with a brush
and requires thinning with denatured alcohol before it can be brushed.
This finish also requires that you work fast with your brush and
maintain a wet edge.
It is available in a range of colors from clear
to an orange/amber color. Orange/Amber shellac is known for the warm
tones it gives wood. Shellac breaks down over time, so be sure to
purchase a fresh can, and don’t use anything over a year old.
Lacquer is used most often in furniture factories
because its fast drying properties reduce dust related finish
problems. It is usually applied with a spray gun although “brushing
lacquers” that cure slowly are also available. If you choose to
spray a lacquer finish be aware that the fumes are hazardous to your
health and the dry dust can be explosive.
Because of increasing environmental concerns a
new class of finished has been developed. These “Water-Based”
finishes are often marketed as “polyurethane”, “varnish”, or
“lacquer” which is untrue since all of these finishes are
solvent-based. You can tell if a finish is water based because the can
should mention “water cleanup” as an added feature.
“Water-based” finishes are basically latex paint without a
pigment. The two primary downsides are that they tend to hold visible
brush marks and are less durable than the more conventional finished.
They also tend to bubble with brushing. You’ll want to use a
synthetic bristle brush and try not to over-brush the surface. Also
try to avoid temperature and moisture ranges that exceed indoor
conditions. Moisture or temperature ranges on either end of the scale
can cause drying problems.
Properly preparing your finish between coats is
an important step. With finishes that require building to achieve a
thick coat it is extremely important to sand with fine sandpaper
between coats to remove bumps and high spots. Wet sanding with special
“wet sanding” paper is an excellent way to smooth out the surface
without creating lots of airborne dust, which could land, in your next
coat of finish. The best advice we can offer is to test your finishing
techniques on a piece of scrap wood before you move on to your
recently completed masterpiece.
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