Popular Woodworking 2005-11 № 151, страница 77

TESTING FOR OIL: DOES IT GET HARD?

With all of the confused labeling from manufacturers, how can you tell if a finish is wiping varnish? It's simple. If the finish meets these three criteria, it's wiping varnish:

• It thins and cleans up with paint thinner. ("Mineral spirits," "petroleum distillate" or "aromatic hydrocarbon" will be listed on the container.)

• A puddle on top of the can, or on any non-porous surface such as glass, gets hard within a day or two.

• It is watery thin. (Full-strength varnish and polyurethane meet the first two criteria, but they are relatively thick, like syrup. They are also labeled "varnish" or "polyurethane," and wiping varnish is not.)

Curing hard is the critical characteristic because it makes it possible to leave each coat of wiping varnish wet on the surface - as wet and thick as you want as long as the finish doesn't puddle or run. You can build coats one on top of another to achieve a thicker coating for better protection of the wood against moisture.

Confusion is caused because wiping varnish is often sold or marketed as "oil," and it is sometimes included in the same category as oil in books and magazine articles. But oil doesn't cure hard unless you leave it for many months or years, and then only if it is applied very thin. So all the excess oil has to be wiped off after each application or you will end up with a sticky, gummy mess. Oil is about as different from wiping varnish as any finish can be.

It's true that oil is used as an ingredient in the making of varnish, but remember that once the oil and resin have been cooked, they are no longer oil or resin. (See page 72.) I like to compare what happens in varnish to bread. Once you add water to flour and yeast to make bread, you can't go back to flour or yeast, and it would be foolish to call the bread "flour" or "yeast." Likewise, it is totally inaccurate to call thinned varnish "oil" or "resin."

There are two categories of oil: natural oil, and mixtures of oil and varnish. The two common natural oils that can be used successfully as wood finishes are linseed oil and tung oil. Linseed oil is pressed from the seeds of the flax plant. Tung oil is pressed from

the nuts of the tung tree. This tree is native to China but is now also grown in Argentina and the U.S. Gulf States.

Linseed oil is sold as "boiled" linseed oil when "driers" are added to cause the oil to cure faster (overnight in a warm room when the excess is wiped off). Without the driers, "raw" linseed oil takes weeks or months to cure, even when all the excess is wiped off. With or without the driers, the oil still cures soft, the same as tung oil.

Both linseed oil and tung oil can be mixed with any type of varnish, and the mixture can be thinned with mineral spirits or turpentine. Many brands of these mixtures are sold in home centers, paint stores and in woodworking stores and catalogs.

It's important to stress that these are mixtures or blends of oil and varnish. They are different than varnish itself, which is oil and a hard resin that are cooked together.

Because of the oil included in these mixtures, oil/varnish blends don't cure hard. So you have to wipe off all the excess after each application just as you do with linseed oil and tung oil. Otherwise, you will end up with a sticky, gummy surface.

Recently, I saw a woodworking magazine article that compared a large number of "wipe-on" finishes for characteristics such as viscosity, dry time, penetration and solids content (ratio of finish to thinner). Some of the finishes in the comparison were wiping varnishes. Others were oil/varnish blends.

The article was virtually useless as an aid to choosing a finish because the characteristic that matters most, "Does it cure hard?" wasn't included. You will never make sense of "wipe-on" finishes and overcome the confusion caused by misleading labeling until you understand this distinction.

If a finish thins with mineral spirits, cures hard, and is watery thin, it is wiping varnish. If a finish thins with mineral spirits, is in liquid form (it isn't wax), and cures soft, it is oil or a mixture of oil and varnish. There aren't any other possibilities for a finish that thins with mineral spirits. - BF

Tung Oil Finish

These are examples of oil/varnish blends, simply mixtures of linseed oil or tung oil and varnish, usually thinned with mineral spirits. You can tell that a finish is an oil/varnish blend if it contains mineral spirits and a puddle on top of the can or other non-porous surface takes days or weeks to cure to a very soft and usually wrinkled film.

more you increase the possibility ofbrush marks and bubbles curing in the finish. You also increase the amount of dust that can stick to the finish because a thicker film (after the thinner evaporates) takes longer to cure.

I suggest you begin with one part thinner to two or three parts varnish, and see how it feels to you. You can always adjust the ratio as you are applying the finish.

Applying Wiping Varnish

There are three good methods of applying wiping varnish: wipe off all the excess; wipe off most of the excess; leave the excess.

No matter which method you use, you need to prepare the wood first by sanding out all the machine marks and other flaws. The finish won't disguise them; it may highlight them.

In most cases begin sanding by hand or with a power sander using #100- or #120-grit sandpaper. Sand through all the "wash-boarding" left by jointers, planers, routers and shapers, and sand out any other problems.

Then sand with #150- or #180-grit sandpaper. If you use a power sander - for example, a random-orbit sander - it's a good idea to finish off by hand-sanding in the direction of the wood grain using the same grit or one numbered grit finer to remove "squigglies."

Of the three application methods the most foolproof is to apply the finish just like you do an oil finish. Wipe or brush the finish onto the wood, keep it wet for a few minutes until no more dry spots develop, then wipe off the excess with a cloth, leaving the surface just barely damp.

Let the finish cure for four hours to overnight depending on how warm your shop is, then sand the finish smooth using #280 or finer grit sandpaper. I usually use #320 or #400 stearated (gray)

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Popular Woodworking November 2005