By Sal Marino
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 poly foam brush. That's right, the disposable type. Oil
based varnish is classified as a cold finish. This means the solvent used
is not as strong as evaporative finishes like shellac and lacquer.
Alcohol and lacquer thinner will melt a foam brush but 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 Finish
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 finish through a paper
paint strainer or stocking into another can or jar, then add the same
amount of solvent into the finish. 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,
brush about 1" into the mixture. 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.
Applying The Coats
It's a good idea to also thin out your normal coats a little. You
can reduce each coat 20 to 25 percent 4 parts finish to 1 part
of solvent or 3 parts finish to 1 part
of solvent. This will not effect the strength of the
finish, 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
finish has dried. Prepare the finish by mixing and straining in the
same way you prepared the sealer. Apply the finish 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 finish alone, do not do a
lot of brushing, this will make the solvent evaporate quicker and the
finish will set up too quickly and not have enough time to flow out.
Let the finish dry overnight, and then sand with 320 grit sandpaper and
remove dust using vacuum or tack cloth. When sanding, if the finish
starts to clog the paper, it has not dried enough. If the finish turns
to powder, it is dry enough to sand and apply the next coat. Continue to
apply 2 to 3 more coats of finish 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 finish 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.
Why Do I Get Bubbles and Brush Marks In My
far, one of the most frequent problems readers e-mail me about is how to
eliminate the bubbles in a finish that they are brushing. So let's deal with this
issue. I will be repeating some of
what we went over above.
What Causes the Problems
It is very frustrating when one spends
hours and hours building a project,
making sure that all the joints fit
properly, the wood grain matches from
board to board, removing any glue
squeeze out, etc. only to wind up with
brush marks and or bubbles in the finish
they have brushed on. Most people
immediately blame the brand of finish
they have used and swear never to use
that inferior product again. In truth,
the problems are not with the brand of
finish. Most popular brands of finishes will give you excellent results if you know how to
properly brush on that finish and a little about how finishes flow out
and their set up time.
There are two main factors that cause a finish to bubble and show brush
marks after it has dried. The first has to do directly with the way you
apply the finish, (your brushing method). Many people assume that if
they brush out the finish well, it will level and stay smooth. This is
absolutely incorrect. When you apply a finish by brush, you are
basically agitating (moving the finish around) Brushing introduces air
into the finish, thus creating air bubbles. Even if one is careful, one
cannot completely avoid the development of some bubbles, but if you
brush too much, you will wind up with a whole surface full of bubbles. Secondly,
the more you brush out a finish, the quicker the solvent in that finish
will evaporate, thus the quicker it will set up. If the finish sets up
too quickly, it will not have enough time to level itself. The bubbles
will not have enough time to pop and any brush marks will not have
enough time to flow out.
How To Prevent the Problems
Thinning the finish will make it flow out better so brush marks will
level and air bubbles will have enough time to burst before the finish
sets up. First, make sure the finish has enough solvent in it. Most oil
based varnishes can be thinned with pure gum turpentine or a good
quality mineral spirits, but it is always a good idea to check the label
on the can to make sure. I usually like to thin out the finish about 20
to 25 percent with solvent. This breaks down to 1 part solvent to 4
parts finish or 1 part solvent to 3 parts finish. Thinning this much may
not be necessary when the can is brand new and it's the first time you
have opened it, but each time you use the finish, it will get thicker
because as the amount of finish gets lower in the can, the more air
stays in the can and this will make the solvent evaporate much quicker.
If you are not in the business, I do not recommend buying these finishes
in large cans like gallon sizes. Purchase in smaller sizes or when first
purchased, transfer the finish from the larger can to smaller ones and
make sure the lid is placed on tightly. This will keep the finish from
thickening too quickly.
Reducing the finish with solvent will not make it weaker or effect the
way the finish performs. The only drawback is that when the finish dries
and all the solvent has evaporated, less of the resin (finish itself)
will be left on the surface. Therefore, you will have to apply a few
more coats than you would have if you used the finish without thinning
it. I believe this is well worth the extra time, considering the results
you will obtain.
Proper brushing technique will reduce the amount of air bubbles that
develop in the finish. When brushing on the finish, make sure to load
the brush enough so you can apply a liberal amount of finish without
having to press down on the brush to get more out. Lightly touch the
surface with the loaded brush and then start to move the brush across
the surface. As soon as the brush stops applying a continuous flow of
finish, re-load the brush, in this way you will prevent over-brushing.
Try not to overload the brush, because if you do, you will wind up with
drips or too much material on the surface. Some practice is required
here. Run tests on scrap panels until you get the feel. Once the whole
surface has been covered with finish, then you must perform a method
called tipping off the finish. Do not re-load the brush. Using the very
tips of the brush, take light passes just slightly overlapping each
brush stroke. This should burst the majority of bubbles. This will not
burst all the bubbles, but don't worry, the remaining bubbles will burst
and level because of the extra solvent you added to the finish.
Remember, sand lightly between coats with 320 grit paper.