EVERYTHING YOU NEED TO KNOW ABOUT FINISHES
Master that last crucial step in furniture building
with this complete guide
>From hand-cut dovetails to screw-together joinery, amateur woodworkers and DIY
>enthusiasts use a wide range of skills. But there's a common line every project
must cross before it's done: the finish line. Finishes are necessary to protect
and beautify most woodworking projects, yet the choices can be bewildering
and application methods differ from one product to the next. Getting the finish
right is no chance affair.
I've met many a hobbyist who had built a stunning project but was
terrified to embark on the finishing journey, and sometimes with good reason. It
takes just one failed attempt to make the "unfinished" look like a tempting
option, but a little knowledge goes a long way.
Although the many names of finishing products can be confusing, most
belong to one of four groups: straight oils, oil/varnish blends, varnish and
water-based
finishes. Each has its own characteristics and application methods. Most
finishes fall into one of these groups, with the exception of shellac and
lacquer.
Look for the
characteristics of each group.
As for application methods, all of these products can be applied with ordinary
hand-finishing methods. For the perfectionists among you, see
"Finishing the Finish".
With a little extra time and elbow grease, you can make the difference between
a good finish and an outstanding finish.
TECHNIQUE
1. Straight oils and oil/varnish blends go on evenly and easily with a foam
roller. Wipe off the excess finish in the direction of the grain with a rag.
2. Use a natural-bristle brush to apply varnish or polyurethane. Allow at least
a half to a full day of drying time.
3. A foam brush picks up a good quantity of a water-based finish, but applies
only as much product as you apply pressure on the brush.
4. Apply your last coat of varnish with friction by using a rubbing pad so you
avoid leaving brush marks.
Straight Oils
The straight oils category includes finishes such as pure tung oil, raw and
boiled linseed oil, mineral oil and walnut oil. These are penetrating finishes:
they don't build a film on the wood. Although oil finishes don't provide any
real scratch or scuff resistance, the lack of film can be an advantage. On
a cutting board, for example, a film finish would crack and peel. Small bits of
finish would mix with your food, raising questions of food safety.
Choose a straight oil primarily for heavy-use items that are subject to
cutting and pounding. This includes cutting boards, butcher's blocks, knife
blocks and workbenches. These kinds of items can be recoated with more finish
as needed. My workbench is finished with many coats of tung oil, and I add
another coat each year when I close my shop down over Christmas.
Straight oils are also suited for decorative tems that won't see any abuse
at all, such as sculpture or turnings. Oil finishes leave the wood looking
natural, with a matte finish-wonderful on "art" pieces. But they aren't
protective enough for most furniture, and can really collect dirt and grime on
regularly handled items.
For food contact safety on projects such as cutting boards, stick to tung,
walnut or mineral oil. Walnut and mineral oil will remain greasy to the
touch because they don't fully cure, but they are food-safe. Tung oil isn't
pure unless the can says "pure" or "100%." It's also food-safe; each coat needs
several days to dry. Don't forget about nut allergies with respect to walnut
and tung oil; tung oil comes from a Chinese nut. Where nut allergies are an
issue, mineral oil is the safest choice for food contact. You can pick up a
bottle of light mineral oil from your local drugstore.
Raw and boiled linseed oil are also frequently used for decorative items,
or sometimes as a first coat underneath other finishes, such as oil-based
poly-urethane. Raw linseed oil isn't often used since it can take a full week
to dry. But boiled linseed oil has added metallic driers that speed up the
drying time.
Applying oil finishes is easy: simply swab on a liberal amount with a rag
in any direction you want. Circles and figure eights are fine. Use a foam
roller to cover large areas. Let the oil soak in for about half an hour (they
all dry slowly) and then wipe off the excess with a cloth, rubbing in the
direction of the wood grain.
In the case of curing oils, such as linseed and tung oil, wait until the finish
dries fully, then add the next coat. It will take at least five coats to
see any sort of a sheen, but you can apply as many coats as you want. For
non-curing oils, such as walnut and mineral oil, you can apply a second or third
coat if you want the wood to soak up more; otherwise, just one or two coats
will do.
When you apply oil to cutting boards and salad bowls, the oil will
disappear after several washings with soap and water, so keep a bottle of the
oil
under the kitchen sink and add another coat whenever the wood looks dry.
Oil/Varnish Blends
Danish oil, also called "oil/varnish blend," is made by mixing a small quantity
of varnish with a larger quantity of a curing straight oil, such as tung
or linseed oil. It's thinned with mineral spirits (i.e., paint thinner) and
metallic driers speed up drying time.
By mixing an oil finish, which is easy to apply, with varnish, which is
harder to apply but more protective, you get the best of both worlds. There's
more protection than a straight oil finish because each coat builds a thin
film. You can add to the protection by buying an off-the-shelf Danish oil and
adding varnish, such as oil-based polyurethane. I suggest a mix of 25 per cent
poly to 75 per cent Danish oil. Don't add more than 25 per cent poly: the
mix will be harder to apply.
An oil/varnish blend is great for everything except high-wear items. It
isn't suitable for a kitchen table, for example, which will see lots of spills
and abuse. But it's a beautiful finish for projects such as gently used coffee
or end tables, beds, night tables or mirrors.
Applying these blends is as simple as applying straight oils: you just
wipe on, then wipe off. The difference is that the finish should be left for
just 10 to 15 minutes before wiping off the excess. The varnish component
becomes gummy if it's left thick for too long. Allow at least 12 hours of drying
time between coats; 24 hours is better. Keep in mind that colder temperatures
and higher humidity increase drying time for all finishes.
Varnish
Varnish is made by heating a straight oil with resin. Natural resins exist, but
most varnish is made with synthetic resins, such as polyurethane or alkyd.
Even products sold as "polyurethane" are generally alkyd-modified polyurethane,
which means it's alkyd varnish with poly added for extra toughness.
Spar or marine varnish is like regular varnish except the oil component is
higher, making it more flexible. This is important outdoors, where humidity
fluctuations make wood move. Use spar varnish where you want ultimate
protection from water. UV inhibitors are included too, which prolongs the life
of
your outdoor furniture.
Wiping varnish is nothing more than ordinary varnish thinned with mineral
spirits. When thinned, you can apply it by rag instead of by brush, eliminating
brush marks. Each coat will be much thinner, so you'll need more coats. You can
make your own wiping varnish by buying oil-based polyurethane and adding
one or two parts of paint thinner. Make it thin enough to wipe on easily,
although if it's too thin you'll have to apply more coats.
To apply wiping varnish, rub a rag with the grain direction. Work in a
long stroke from one end to the other, then start the next stroke. Apply as
evenly as you can and then leave it alone. Unlike oil/varnish blends or
straight oils, you have to be careful about the direction in which you apply
varnish
because you don't wipe off the excess. Varnish dries hard in a thicker film.
You can't wipe it later because the varnish sets up quickly and becomes gummy.
For ordinary brush-on varnish, including spar, thinning the product is the
key to good results without defects, such as brush marks and air bubbles-these
surface marks have time to level out. In my experience, the thinner you go, the
more coats you'll need, but the better the results. I thin the first coat
of varnish-the sealer coat that soaks into the wood and fills the pores-25 to
50 per cent. For the next two or three coats, thin the product just 15 to
25 per cent to get a quicker build-up.
Varnish should be brushed on with long strokes, moving from the middle of
the board to the ends. Don't brush into the ends or you'll get a run over
the edge. Once you've covered an area, don't go back unless it's just a minute
later (e.g., to remove a hair) or you'll make it worse. Deal with defects
later.
FINISH OPTIONS
STRAIGHT OILS
(includes pure tung oil, raw linseed oil, boiled linsee oil, mineral oil and
walnut oil)
OIL/VARNISH BLENDS
(sometimes known as Danish oil or Scandinavian oil)
VARNISH
(always oil-based, such as alkyd varnish, spar or marine varnish, polyurethane
or wiping varnish)
WATER BASE
(includes latex urethane acrylic finish-sometimes called water-based poly or
water-based varnish)
APPLICATION DIFFICULTY LEVEL
LOW
LOW
MODERATE
HIGH
WEAR AND SCUFF RESISTANCE
LOW DUE TO THIN FILM
MODERATE IF MANY COATS USED
HIGH
HIGH
HEAT RESISTANCE
MODERATE
MODERATE
HIGH
LOW
WATER RESISTANCE
LOW
LOW TO MODERATE
HIGH
MODERATE
DUST PROBLEMS IN FINISH COAT
LOW
LOW
HIGH
MODERATE
HEALTH SAFETY IN USE
MODERATE TO HIGH
MODERATE
MODERATE
HIGH
AMBERING COLOUR
MODERATE
MODERATE
HIGH ESP. OVER TIME
NONE
DRY TIME BEFORE NEXT COAT
LONG
MODERATE
SHORT TO MODERATE
VERY SHORT
After each coat dries fully, sand lightly with 320-grit paper or higher (400 or
600) to remove imperfections such as dust nibs. The light scratches also
give the next coat a mechanical bond. If you stained the wood prior to
finishing, don't sand until after you've applied two coats of varnish or you'll
sand through and remove the stain.
While three coats of varnish are usually enough, high-wear areas such as
table tops call for four or five coats, especially if you're thinning the
varnish a lot. I apply a minimum of three coats by brush, then I remove defects
with sandpaper, then I add one final coat of wiping varnish, which goes
on very smoothly.
You can use varnish on just about anything except items for cutting or
pounding-the finish film will crack and flake off. Varnish excels on high-wear
items. Look at the
"Finish Options" table
for details.
Water-based Finishes
There's only one true water-based finish. If the can says, "Wash your brushes
with soap and water," then the finish is water base. These finishes are always
milky white in colour and turn clear as they dry. Water-based products are
marketed under different names. Sometimes they are called "water-based varnish"
or "water-based polyurethane."
FINISHING THE FINISH
1. Use mineral spirits and 400-grit sandpaper to level the finish and remove
dust nibs from your project.
2. Use a 50/50 mixture of mineral spirits and mineral oil and throw in pumice
to form a moist slurry. Wipe on the mixture with the grain. After a few minutes
of rubbing, remove the slurry with clean rags and paint thinner. You'll have a
beautiful satin sheen that's impossible to get from a can. For a higher
sheen, repeat with rottenstone.
3. Place a ball of paste finishing wax in a cotton rag to serve as a finishing
pad.
4. Rubbing friction lets a thin layer of wax melt through, but controls the
amount. Buff out with a clean rag.
As shown in the table, water-based products have excellent wear and scuff
resistance, which is why they are used on wood flooring. But water-based
finishes don't have the other highly resistant properties of traditional
varnish. Still, use a water base for a protective finish where high-performance
water and heat resistance don't matter. These finishes are also healthier to
use, since they're primarily thinned with water, not a strong solvent.
Water-based finishes are unique in that they're absolutely clear. Other
finishes create an amber tone. If you want a clear finish, water base is your
only option. You'll also use a water base over light stain or paint, such as
white, cream, peach or even pickled wood. Oil-based products add too much
colour, completely changing the tone of light stains.
To apply water base, brush on the finish in long strokes using a synthetic
(poly or nylon) brush. Unlike varnish, you have no time to waste. The product
dries quickly, and if you over-brush you'll drag up semi-dried finish. Cover
well in one stroke and move on; sand out problem areas later.
Because water-based finishes dry fast, dust nibs aren't as much of a
problem. But the cured film will still benefit from a rubbing schedule after
you're
done (see right, "Finishing the Finish"). Quick drying time also makes brushing
the finish on more difficult, so you'll really have to move fast. Mastering
oil-based varnish first will get you ready to work with water-based finishes.
Handling water-based finishes is key. They are very temperature-sensitive,
so don't use them under 23ºC. The air has to be that temperature, as does
your project and the can of finish. If the can has been stored in a cool area,
lower the entire can about 3/4" into a pail of warm water for about half
an hour before you start. Never try to heat a finish on a stove or open flame.
Water-based finishes shouldn't be thinned like oil-based varnishes. They
have a complex formula that allows a solvent-based finish to be dispersed
in water to increase safety to the user. But it's a fragile formula that can be
disturbed by excess thinning. As a general rule, use distilled water and
don't thin by more than 10 per cent.
Test and Have Fun
As daunting as fine finishing can be, experience is the best way to learn.
There are a variety of finishes, so experiment with them all. The best advice
is to test your chosen finish on scrap wood first. Use the same species as your
project, sanded to the same grit level. Then you'll know exactly what to
expect. Happy finishing!
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