of the pores. Little puddles form soon after you have removed the excess finish. Continue to wipe these puddles dry until they stop forming. Go back over the wood once even hour or so with a drv

cloth until the bleeding stops. (See sidebar. A Remedy for Bleeding, page 50.)

6. Apply additional coats. Allow the first coat to cure overnight. Smooth any roughness that remains by rubbing with 00(H) steel wool or sanding with 280-grit or finer paper. Apply the next coat. (You can combine the two steps by sanding the wood while it's wet with the second coat of finish.) Wipe the wood dry. Apply as manv coats as you want, allow-

ing at least one day's drying time between each coat. There's no point in applying more than three or four coats unless you're trying to build a thicker film with wiping varnish or polymerized oil.

7. Build the finish. If you're using wiping varnish or polymerized oil. you may want to build the

finish to a thicker film to give the wood better protection or to increase the appearance of depth. To do this, leave each coat wet on the wood's surface—that is, don't wipe off all the excess. Make sure that the space you're working in is relatively dust free. If dust sticks to the finish, sand it lightly between coats with 320-grit or 400-grit sandpaper to remove the dust nibs. Use stearated (gray) sandpaper dry or wet-dry (black) sandpaper with mineral spirits as a lubricant. Be careful not to sand through the thin finish.

8. Apply the final coat. No matter which "oil" finish you use, you can make the final coat smoother to the touch and raise or lower the sheen. To smooth the last coat of straight oil or oil-varnish blend, sand it wet with very fine sandpaper (400-grit or finer), and then wipe off the excess.

To smooth and increase the sheen of anv finish, rub the last

coat very hard with a dry cloth or rub the cured finish with a commercial rubbing compound.

To smooth and reduce the sheen of any finish, rub the last cured coat with 0000 steel wool. You can lubricate the steel wool with mineral spirits, a non-curing oil, or paste wax to reduce scratching.—B.F.

Linseed oil—Linseed oil is extracted from the seeds of the flax plant. Raw linseed oil is an inefficient finish because it takes many days to cure. To make it more effective, metallic driers are added to the oil. These driers are usually salts of cobalt, manganese, and zinc. The>f act as catalysts to speed the curing. (Lead is no longer used as a drier because it causes health problems.) Linseed oil with driers added is called boiled linseed oil and cures in about a day. Boiled linseed oil is not made by boiling raw linseed oil.

Linseed oil protects wood less than any other type of finish except wax. It's a soft, thin finish, and provides little significant barrier against scratching. It's also easily penetrated by water and water vapor. Water will work through a linseed-oil finish and cause a smudge in five to 10 seconds. Water vapor will pass through a linseed-oil finish almost as

A eta*** v.txxl tnésh Irrproves tt* Bow aroqtassot oé based pant


Linseed Oil

A eta*** v.txxl tnésh Irrproves tt* Bow aroqtassot oé based pant mumei

if it weren't there.

It's important to understand the consequences of water-vapor exchange. When the humidity is high, wood takes on moisture from the air and swells. When the humidity is low, wood releases moisture and shrinks. Expansion and contraction brought on by weather changes accelerate splitting, warping and joint failure. Because one of the primary purposes of a finish is to slow water-vapor exchange to reduce stresses in the wood, linseed oil is of questionable value as a protective finish.

Tung oil—Tung oil is extracted from the nuts of the tung tree, which is native to China. Tung oil has been used for centuries in China but was not introduced into the West until the very end of the 19th century. Though tung oil is more expensive tlian linseed oil. it has established a firm position in the coatings industry because of its superior water resistance. Many high-quality varnishes are made with tung oil. But, contrary to what you might think, straight tung oil alone is seldom used as a finish.

Five or six coats of straight tung oil make a fairly water-resistant finish, but

Straight oils, like tung oil and boiled linseed oil (left), cure slowly and dry soft, providing no resistance against scratches, water, or water vapor. Though often marketed as an "oil" finish, wiping varnish (below) is regular varnish thinned with mineral spirits. Multiple coats can l>e built up into a relatively thick, protective film.

it's too soft and thin to be resistant to scratches or water vapor. It's also difficult to get a nice-looking finish with straight tung oil. The first three or four coats appear flat and splotchy on the wood and feel rough to the touch. Only after five or six coats, sanding between each coat, can you get an even, satin sheen. But a straight tung-oil finish is never as smooth to the touch as a linseed-oil finish.

Wiping Varnish

Most of the "oil" finishes you see on store shelves arc varnishes that have been thinned enough to be wiped on the wood. Many of these wiping varnishes have names that lead you to believe they're oil finishes. They aren't oil, they're varnish.

Varnish is made by cooking one or more oils widi or synthetic resins (alkyd, phenolic, or polvurethane). Manufacturers use linseed and tung oil. and/or scmicuring oils such as soy bean and safflower oil. When oil and resin arc cooked together, they combine chemically to form an entirely new substance. This new substance—varnish—cures much faster and harder than oil (again, check the overspill around the caps of your cans of varnish). Varnish also cures with a glossy appearance (unless the manufacturer adds flatting agents to give a satin or flat sheen).

Most important is the hardness. Because varnish hardens, multiple coats of wiping varnish can build up a relatively thick film on the surface of the wood. When varnish is built up, it protects the wood from all but the most severe scratches. It also forms an excellent barrier against stains, water, and water-vapor exchange. Remember that it will take many more coats of thin wiping varnish to achieve the same protection as one or two brushed coats of regular varnish.

Wiping varnishes vary in hardness and water resistance, depending on the types and proportions of resins and oil in the varnish. (See "A Tale of Two Finishes," aw, «11, November/December 1989.) But no wiping varnish I know of lists these ingredients on die container. In fact, the words tung oil in the product name arc no guarantee that the varnish was made with tung

tung on.

< lil-varuish blends art* simply u mixture of oil uiul \ arni-li. usually thinned with mineral Spirit* to make application ribirr.

oil. l ung oil seems to have become a synonym for wiping varnish.

For wipe-on/wipe-off applications, the quality of the varnish doesn't matter much because the film is so thin. Hut the quality of the varnish does matter if you intend to build up a thicker film on the surface of the wood. You don't want a soft varnish film on a tablctop. for example. In cases where you want a thicker film, you may want to buy a can of high-quality varnish and thin it with mineral spirits until you can spread it with a cloth.

Oil-Varnish Blends

Oil and varnish (including polyurethane) arc compatible and can be mixed. The resulting blend performs with some of the characteristics of each component.

The oil in the finish reduces the gloss of the varnish and makes the finish cure more slowly. Application is therefore easy because you have plenty of time before the finish gets tacky. But the oil also softens the finish. so you can't build oil-varnish blends to a protective thickness as you can with varnish alone.

The varnish component gives the finish more gloss than straight oil alone. That means with an oil-varnish blend, you can achieve a satin sheen with two coats instead of the three to six it usually takes with straight oil. The varnish also makes the finish more protcctivc than with straight oil alone.

Many woodworkers choose to make their own oil-varnish blends. Though the recipes are often shrouded in secrecy, most of these concoctions are made simply by mixing varnish and/or

Polymerized oil is straight oil that's bppn heated in an oxygen-free environment to make it cure harder and faster.

polyurethane with tung oil and/or boiled linseed oil. The mixture is then thinned with mineral spirits to make application easier.

The higher the varnish-to-oil ratio, the better the resistance to scratches, water, water vapor, and stains. But if you get the percentage of varnish too high, the finish will set up too quick ly, making application difficult. Start with equal proportions of varnish and oil (tung oil and/or boiled linseed oil) and van* the formulation from there.

Using tung oil (and no linseed oil) in the mixture makes the finish significantly more resistant to water. But the higher the ratio of tung oil to varnish, the more coats it will take to achieve an even, satin sheen.

Polymerized Oil

When oil is heated to about 500°F in an oxygen-free environment, it goes through a chemical change, which causes it to cure fast, glossy, and hard, much like varnish. Oil processed in this way is called beat-bodied oil or polymerized oil. Any curing or semicuring oil can be heat treated in this manner to change its characteristics. The primary uses of polymerized oil in the coatings industry arc for making ink and outdoor paints, but polymerized oil can also be used as a finish by itself.

Polymerized oil alone is not widely used as a finish. It's expensive, it cures too fast for easy use on large surfaces, and it doesn't build to a thick film as quickly as varnish does (tiny cracks develop if the coats are too thick).

Distinguishing One Product From Another

Straight oils and polymerized oils are clearly labeled on the container as "boiled linseed oil, l()()-percent tung

You can tell if you have an oil-varnish blend or a wiping varnish by pouring a little of the finish on a piece of gla»rt and letting it cure overnight. Wiping varnish will cure hard and smooth (left). An oil-varnish blend will cure soft and wrinkled (right).


Oil finishes can "bleed" back out of the wood's pores and form puddles around the pore openings. If you allow the finish to cure, it forms glossy scabs, which are difficult to remove without stripping the finish. (See photo.)

Bleeding occurs mostly with large-pored woods and in pans of the wood where the cell structure allows deeper oil penetration. Bleeding no longer occurs once the ¡Hires are sealed (usually alter the first or second coat).

To keep scabs from forming, wipe any bleeding off before it cures. Go back over the wood once every hour

Was this article helpful?

0 0

Post a comment