FINISHES : Natural or Synthetic? Some Hard, Clear Facts

At one time, the term varnish was used for any coating applied as a liquid that dried to a solid film.

Today, varnish is still a catchall term, but it is usually restricted to finishes that contain oil and natural or synthetic resins or, lately, latex and hardeners, such as polyurethane.

The new water-base latex varnishes go on white and dry clear, unlike the older formulations, which had a tendency to turn yellow.

Polyurethane resins are found in just about everything--they are hard and resistant to scratches, abrasions, heat and moisture, and hardly anything will stick to them. For high-traffic surfaces, such as tabletops and floors, it’s hard to beat its durability.

Unfortunately, polyurethane has trouble holding its grip on raw wood, and spot repairs are difficult because an old coat won’t accept a new coat. That’s because polyurethane is so good a repellent that it will repel an additional coat of polyurethane once it has cured.

Oil finishes come next on the list of popular coatings. In the past, furniture was finished with linseed oil. Linseed oil, however, will not ever really dry hard. For that reason, most woodworkers prefer tung oil, Danish oils or other so-called penetrating oils.

Several coats of tung oil, which is usually sold thinned and fortified with drying agents, provide a low-luster satin finish that remains flexible and will resist moisture.

Because tung oil dulls fairly quickly and does not resist scratches well, it’s not the best finish for dining tables or other high-use surfaces.

Penetrating oils may contain tung or linseed oil, but they also are made with resins that harden in the wood. These finishes are actually thin varnishes, therefore. They offer the subtle appearance of an oil finish, yet are generally harder and more resistant to scuffs, abrasions and scratches.

Shellac is an old standby that has found new favor, although it has some drawbacks. It is available in dry form, but most woodworkers today buy it premixed with alcohol.

It has a limited shelf life, though, does not cure well on damp days, and puts up a poor barrier for standing liquids.

Although it is not tougher than polyurethane, shellac will provide a hard finish that often will cling to wood better than a synthetic resin. It brushes on easily and dries with great speed.

Lacquer is probably the finishing product most popular with professionals because it yields a durable protective coating that dries fast.

Lacquer is usually applied with a spray gun (and it’s highly flammable). Brushing lacquers are available to get you around the spray-equipment problem, but these are flammable too. New water-based substitutes are available for spray application.

Now that wax polishes come in spray cans, it’s easy to forget that wax was originally a finish, not a finish dressing. The wax we’re referring to here is paste wax, which is a blend of beeswax and carnauba wax.

Although paste wax can be applied over raw wood, the fact that it offers poor resistance to water means that you should apply a preliminary finish first.