Popular Woodworking 2000-10 № 117, страница 16

THE FINISHES

Even without the three groups to simplify the finishes, there are still only seven types of finishes to choose among:

• Oil, which includes raw and boiled linseed oil, pure tung oil (not the thinned varnishes often sold as "tung oil") and blends of oil and varnish, commonly sold as "Danish oil" and "antique oil." The characteristic that unites these finishes is their inability to cure hard,which makes them functional only if you wipe off the excess after each application.

• Varnish, which includes all hard-curing finishes that thin and clean up with turpentine or a petroleum distillate like paint thinner. Common polyurethane is a varnish, as are wiping varnishes (varnish thinned with paint thinner) and gel finishes.

• Two-part finishes, which, as the name indicates,are finishes that require the combining of two parts to cure.These include pre- and post-catalyzed lacquer (pre-cat-alyzed has the catalyst already added), conversion varnish, polyester, two-part polyurethane (both solvent-base and water-base) and epoxy.

• Shellac, the only finish that thins and cleans up with alcohol.

• Nitrocellulose and CAB-acrylic (water-white) lacquers, which always thin and clean up with lacquer thinner and can be easily dissolved with lacquer thinner even after they're fully cured.

• Wax, which includes liquid, paste and solid,and always dissolves and thins with turpentine or a petroleum distillate.As with oil,you buff off all the excess to make the soft-curing wax functional.

• Water-base finishes, which include all finishes that thin and clean up with water except those that would be classified as "two-part" finishes.

In practice, you rarely have to choose among all seven types.You usually choose among four.

To begin with,you can eliminate wax from the seven (except for decorative objects like turnings and carvings) because of its very weak protective and durability qualities. Wax is more often used as a polish on top of another finish to add shine and scratch resistance.

Of the remaining six types, you're usually limited to oil, varnish, shellac and water base if you aren't using a spray gun, because the other finishes dry too fast to apply with a cloth or brush.

If you are using a spray gun, you normally choose between two-part finishes, shellac, lacquer and water base because you can achieve all the looks and durabilities you want much faster with these finishes.

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3375

Flexner on Finishing

continued from page 24

is a price to pay for protection and durability. Scratch resistance, for example, has the negative side of making a finish difficult to rub to an even sheen using abrasives. Solvent resistance means greater difficulty recoating, repairing and stripping. Heat resistance makes burn-in repairs less successful, and alkali resistance increases the difficulty of cleaning brushes and stripping.

At the sacrifice of better protection and durability, most refinishers and high-end furniture factories use lacquer instead of two-part finishes because of the reduced problems recoating and repairing, and because lacquer is easier to polish to a beautiful satin sheen.

Ease of Application

As a final lesson to be learned from the Tinker-Toy, spaghetti and soccer-ball analogy, consider that finishes are easier to apply the faster they dry and become dust and sag free. Fast-drying shellac, lacquer, catalyzed finish and, to a lesser degree, water-base finish, are less likely to run, sag or collect dust than varnish. If you are one of the many woodworkers who use regular alkyd or polyurethane varnish for your finishing projects, you need to recognize that you're using the most difficult of all finishes to make look nice.

Ease of application is the reason factories that want the ultimate in protection and durability for objects like office furniture and kitchen cabinets use catalyzed finishes instead of polyurethane, even though these finishes are still difficult to rub to an even sheen and to repair and strip. No finish has everything.

Conclusion

It's often difficult to keep all the characteristics in mind when choosing among finishes, especially if you haven't used them all yourself. You can overcome much of your lack of hands-on experience by using the mental pictures of Tinker Toys, spaghetti and soccer balls to help keep the differences straight. PW

Bob Flexner is a nationally known finishing expert in Norman, Oklahoma, and the author of "Understanding Wood Finishing."

12 Popular Woodworking October 2000