By
Sal Marino
For many amateur woodworkers or hobbyists who either cannot
afford spray equipment or do not have enough space in their
shop to set up a spray booth to safely spray finishes like
lacquers, brushing on an oil based varnish is one of the
best choices for a topcoat finish. Oil based varnish does
not dry very quickly, therefore it will take longer to complete
a finishing project using varnish as opposed to lacquer,
water based or other faster drying finishes. However, if
you are an amateur or home woodworker, not a pro who has
to worry about meeting a deadline, time should not be top
priority, quality should. Varnish has very good resistance
against abrasion, wear, heat, solvents and water vapor.
Other than its slow drying time, which can also cause problems
with dust getting trapped in the finish before it dries,
the only other disadvantage to oil based varnish is that
it tends to yellow over time. The dust problem can be taken
care of by setting up a dust free finishing area in your
shop and sealing it off with heavy plastic to reduce the
amount of dust and sawdust in that area. The problem of
yellowing cannot be handled as easily. First, most oil based
varnishes are amber (yellowish) in color to begin with.
This is because the oils used to make the varnish are amber.
Therefore, oil based varnishes tend to somewhat change the color of
the raw or stained wood when they are applied. It is not
a considerable change, and unless applied over a very light
colored or white stain, it is satisfactory. In fact, amber
varnishes actually give darker woods like walnut and mahogany
a warmer appearance. Non-amber or what are called water
white finishes like some lacquers and water based finishes
can leave a cold look on darker colored woods. However,
if needed, there are a few water white varnishes available.
One is called water white restoration varnish manufactured
by H. Behlen & Bro. Nothing can really be done about yellowing
of varnish over time and if you think about it, all film
finishes break down in one way or another over time and
have to be removed and replaced with a new finish. Tung
oil yellows less over time than other oils, therefore a
varnish that contains tung oil will have a tendency to yellow
less over time.
Oil based varnish is manufactured by cooking certain oils that
can cure with resins. Once this blend of cooked oil and
resin is complete, solvents are added to make it thin enough
to apply and metallic dryers are added to help speed up
the curing time. Initially, linseed and tung oil were used
by manufactures for the curing oils and natural resins like
pine and gum resins were used along with solvents like gum
turpentine and mineral spirits (to thin it out) and lead
used for the drier. These ingredients were not only used
to make varnish, but also paint. With the exception of lead,
you can sometimes still find some of these ingredients in
today's oil based varnishes, but modern varnishes usually
use synthetic resins which are superior in strength and
longevity and curing oils that are less expensive to use
in the manufacturing process along with a blend of solvents
and metallic dryers like cobalt and zinc that do not cause
health problems such as lead does.
Types
Of Oil Based Varnishes
Phenolic
Resin
This varnish
is made with phenol (a plastic) and formaldehyde. The phenol
is a solid and is made into liquid by heating it with oil
and then adding in the other ingredients. When the finish
is applied in a thin film and exposed to to the air, the solvent
will evaporate and it will turn back to its solid form.
Alkyd
Resin
Less expensive,
this is a type of polyester resin that is combined with alcohol
and acid. It is also cooked with oil to create a varnish.
This is the most commonly used resin in commercial varnishes
today.
Polyurethane
Yes, that's
right. Polyurethane is classified as an oil based varnish,
although some purists will disagree. Initially developed to
be used as a substitute for other plastics, polyurethane has
become on of the most commonly used resins in the manufacturing
of many wood finishes. Polyurethane is a very tough, abrasion
resistant resin. There are many types and forms of polyurethane,
but the kind of polyurethane finish you are used to seeing
in paint and woodfinishing supply stores is not pure polyurethane,
but rather an alkyd varnish that has been modified by adding
some polyurethane into it. That is why polyurethane should
be classified as a varnish. Perhaps a better description would
be modified varnish, but nevertheless, still varnish. It is
applied and it cures in the same manner as other oil based
varnishes. Contrary to what many people say about polyurethane,
most modern high quality polyurethanes do not dry leaving
a plastic appearance. They are available in various sheens
from satin to semi-gloss to gloss and can also be rubbed to
a beautiful smooth luster. Polyurethane's abrasion resistance
makes it on of the most commonly used finishes today.
Satin Vs.Gloss Or Semi-Gloss
Throughout the years, many pieces of furniture have been finished with
varnishes and other topcoat finishes. Years ago, and on much
custom furniture still today, the varnish was applied and
then rubbed down with steel wool or sandpaper to cut down
the shine and give it a more pleasing look. Today, woodfinishing
manufactures make varnishes in different levels of shine so
the furniture does not have to be rubbed down after the finish
has been applied thus saving many hours of hand or machine
rubbing. These varnishes are sold in different sheens. Some
will give the user a high gloss finish, others like a satin
will have a slight gloss. All topcoat finishes start out as
high gloss and if the manufacture wants to make a satin or
semi-gloss finish, they take the gloss finish and add a flattening
paste into the finish along with the oils resins and other
ingredients we now know are used to make varnishes. This paste
is usually some kind of zinc oxide and it settles to the bottom
of the can. This is why you do not have to stir a gloss varnish
before it is applied, but you must stir satin or semi gloss
varnishes to get the paste off the bottom of the can and mix
it into the finish. The flattening paste makes the finish
a little duller and prevents the light from reflecting off
the surface as much as a gloss finish. The flattening paste
also makes the finish less transparent, thus creating a cloudy
look. If you apply too many coats of satin or semi-gloss varnish,
you could actually start to obscure the grain of the wood.
Whenever I elect to use a satin or semi-gloss varnish, I will
use gloss varnish and then only on the last one or two coats
use the satin or semi-gloss, this way I can keep the clarity
and still achieve the desired sheen. Satin and semi-gloss
varnishes are also softer than gloss varnishes because the
flattening paste or agent used will soften the film finish.
If you need a really hard abrasion resistant finish, but want
a satin or semi-gloss sheen, it's best to use a gloss and
after it has cured, rub it down with steel wool or other fine
abrasives. This will also give you a smoother surface, removing
any dust nibs and leveling the surface.
What
Varnish Should I Use For My Job?
To determine
what type of varnish you should use for a particular job,
you must look at what type of oils and resins are contained
within a varnish and what the ratio of oil to resin is. Varnishes
that contain a larger amount of oil to resin are called long
oil varnishes. Varnishes that contain a lower amount of oil
are called medium oil varnishes. Long oil varnishes are more
flexible than medium oil, but also softer. Medium oil varnishes
are harder, but are more brittle. For exterior use, a long
oil varnish is best. Because it is more flexible, the varnish
will expand and contract with the wood as changes in temperature
and humidity take place. A medium oil varnish will not move
as much and therefore as the wood moves and the varnish does
not, the varnish will soon start to crack and peel. Medium
oil varnishes are best used indoors where a lot of wood movement
does not occur and a harder finish is desired. The resin contained
in a varnish is also important in determining what type to
use for your project. Some resins are more elastic than others,
making them best suited for exterior uses. Phenolic resin
is more elastic than other resins, therefore it will be able
to withstand the extreme wood movement of exterior projects
without quickly breaking down and cracking. Alkyd and polyurethanes
are better suited for interior use. Not as important, but
still a factor is what type of oil is used. Tung oil is more
water resistant than linseed or other oils, therefore it would
be a better oil for exterior use, but much more expensive.
Putting this all together, we basically come down to two categories,
interior and exterior use. For exterior use, a modern spar
varnish which is long oil and is made up of tung oil, Phenolic
resins, solvents, dryers an Ultra Violet blockers (to protect
the color of the wood from fading) is probably your best
choice if you elect to use an oil varnish outside. Although
Spar varnishes have a tendency of initially being more amber
(yellow) in color because of the color of the Phenolic resin.
For interior use, my favorite is polyurethane modified varnish.
The best I used is a product called Wood Glo, it is a satin
poly that flows out beautifully and lasts decades. It is
sold by Constantine's in New York (See Sources) back on
my homepage. They also have a gloss version called Super
Shield.
Brushing On Varnish
While not very easy to apply by spray application, oil based varnish
is one of the easiest finishes to apply by brush. Because
varnish sets-up slowly it gives the user plenty of time to
brush and spread it out evenly on to the surface. It's hard
to spray because it has a tendency to run if applied too heavy.
I firmly believe that any film finish can be sprayed successfully
if thinned out enough, but varnish is one of the last finishes
I would want to spray. Over many years of testing, I have
come to realize that brushing is the best way to apply oil
based varnish.
Before applying varnish by brush, you should know a little more
about how long it takes for each coat to set-up and how
long before you can apply the next coat along with how it
reacts to temperature and humidity and some other facts.
Oil based varnish is much higher in solids than some other
film finishes like lacquer. Therefore, it should only take
a few coats of varnish to build a film significant enough
to protect the surface of what you are finishing. After
the surface has been sealed, it usually only takes about
three coats to give you enough protection.
One very important factor when applying varnish is how the temperature
effects the speed at which it cures. You should not apply
varnish in temperatures lower than 65 degrees. If you apply
varnish in lower temperatures it may take several days,
even weeks for it to cure. Room Temp. (approx. 70 to 75
degrees) is good for applying varnish. Hotter temps. will
make the varnish cure quicker, but the solvent in the varnish
will evaporate quicker, making the varnish set-up quickly
and you may have a problem getting the varnish to flow out
properly. This could result in brush marks, bubbles and
an uneven film. When working in temperatures higher than
75 degrees, try not to work on large surfaces.
Some Tips For Preparation
Try to set aside a room or part of your work shop to apply your varnish.
This room should be as dust free as possible. Do not do any
other woodworking, (especially sanding) in this area. If you
are going to set aside an area of your shop instead of using
a different room, it would be a good idea to also surround
this area with heavy plastic sheeting. Before applying the
varnish, wet mop the floor, this prevents you from kicking
up any dust when you walk around. I always place clean craft
(brown) paper under the piece I will be varnishing. Once the
surface has been prepared properly you are ready to brush
on your varnish.
Choosing A Brush
There are
a number of high quality brushes that can be used for brushing
on clear topcoats. The best for shellac and lacquers are natural
hair (like badger) or china bristle brushes. While any of
these brushes will do a great job when applying varnish, there
is a much less expensive alternative. A polyfoam brush. That's
right, the disposable type. Oil based varnish is classified
as a cold finish. This means the solvent use is not as strong
as evaporative finishes like shellac and lacquer. Alcohol
and lacquer thinner will melt a foam brush but the mineral
spirits, solvents or turpentine used in most oil based varnishes
will not harm a foam brush. Foam brushes are especially useful
for novices who have a hard time getting brush marks out when
applying a finish. If used properly, you can get excellent
results. I always have a good supply of 1",2" and 3" foam
brushes in my shop. They are very inexpensive, so I use one
for each coat and then throw it away.
Applying The Varnish
Sealer
Coats
You don't need a special sealer to seal the wood. Special sealers like
sanding sealers will not do any better of a job of sealing
the wood than the finish itself. Sealers only make the first
coat easier to sand, thus speeding up production time. Also,
if you use the wrong type of sealer, you may have adhesion
problems. The best sealer for your first few coats should
be the varnish itself. Take some of the same varnish you are
planning to use as your finish and thin it down 50 percent
( this is a 1 to 1 ratio) with mineral spirits or gum turpentine.
This will be your sealer. It will do a good job of sealing
the wood and you won't have to worry about contamination problems.
Pour some varnish through a paper paint strainer or stocking
into another can or jar, then add the same amount of mineral
spirits into the varnish. Stir well and strain a second time
into a deep dish or bowl. It's best to work out of an open
bowl or dish so you can easily dip your brush into it. Now,
dip the foam brush into the mixture until the brush has been
loaded slightly past the bevel on the foam brush. Lift the
brush up and let the excess drip back into the dish. Next,
brush on the first coat with the grain making sure not to
leave any puddles or drips. Allow the sealer coat to dry overnight
and then sand with 320 grit paper. Remove the dust with a
vacuum, or tack cloth. If you are working on very porous woods,
apply a second sealer coat following the previous steps.
Varnish Coats
It's a good idea to also thin out your coats of varnish a little.
You can reduce your varnish 20 to 25 percent 4 parts varnish
to 1 part mineral spirits or gum turpentine or 3 parts varnish
to 1 mineral spirits or gum turpentine. This will not effect
the strength of the varnish, it will only make it flow better
and allow time for air bubbles that form when brushing to
pop. The only drawback is that you will have to add a few
more coats because less will remain on the surface once the
varnish has dried. Prepare the varnish by mixing and straining
in the same way you prepared the sealer. Use a foam brush
and load it in the same manner as the sealer. Apply the varnish
to the surface by brushing either with or against the grain
initially. The main idea is to get it on the surface doing
as little brushing as possible. Once on the surface take one
light pass with the tip of the brush moving with the grain.
Overlap each pass slightly, then leave the varnish alone,
do not do a lot of brushing, this will make the solvent evaporate
quicker and the varnish will set up too quickly and not have
enough time to flow out. Let the varnish dry overnight, and
then sand with 320 grit sandpaper and remove dust using vacuum
or tack cloth. When sanding, if the varnish starts to clog
the paper, it has not dried enough. If the varnish turns to
powder, it is dry enough to sand and apply the next coat.
Continue to apply 2 to 3 more coats of varnish using the same
process. If you are going to rub out the finish (by wet sanding)
after it has cured, you may want to apply at least a total
of 6 coats ( not including sealer coats). This is because
if there is not enough varnish left on the surface, you may
cut through the finish into the raw wood in some spots. Once
you have applied the last coat, let the finish cure for several
weeks before you are ready to use it or rub it out. Varnish
does not need much maintenance. If you wish, you may apply
a coat of paste wax or liquid polish from time to time.
Sal
Marino is a woodworker and a finishing expert from New
York. He works for Constantine's. He teaches woodworking classes
at Constantine's, offers private instruction, lectures around
the United States and is a finishing consultant to many woodworking
and restoration companies. He is also the author of numerous
woodworking articles.
