Mention the word "varnish" and most people think of polyurethane. Polyurethane varnishes are made by reacting a vegetable oil (linseed, soya or safflower to name a few) with polyhydric alcohol and diisocynate. A polyurethane finish is tough and resistant to household detergents and boiling water. In spite of these advantages, there are still compelling reasons to choose traditional oil varnish.
Many furniture makers and refinishers find that polyurethane finish has an artificial quality that makes furniture look more like plastic. Most wood furniture, particularly period reproductions, looks more natural when finished with traditional varnish.The term "varnish" has often been used to describe any transparent finish that produces an elastic film, but, strictly speaking, varnish is a "resinous solution." Most natural resins are produced by plants or trees to cover a wound. One familiar resin, rosin, comes from coniferous trees like pine, fir and balsam. Other resins include anime, benzoin, copal, damar, gum arabic, mastic, sandarac and shellac (shellac, however, is secreted from an insect, not a plant).
Manufacturers have found that using natural resins can be expensive because harvesting them takes time, and they are often rare and hard to find. Today almost all manufacturers use synthetic alkyd resins to make oil varnish.
The resin, either synthetic or natural, must be dissolved in a solvent. Before the development of modern oil varnishes, spirit varnishes using shellac, damar or mastic resins dissolved in alcohol were popular. The finish produced by spirit varnishes was transparent but brittle and susceptible to water damage.
Varnishes fall into three groups based on the proportions of oil and resin: long oil, medium oil and short oil varnishes.
Long oil varnish has 50 to 100 gallons of oil for every 100 pounds of resin. Long oil varnish produces an elastic and water-resistant film that is ideal for exterior or marine applications. The film is too soft to be hand-rubbed, so it is not a good choice for furniture. Spar varnish is a typical long varnish.
Medium oil varnish has 12 to 40 gallons of oil for every 100 pounds of resin. It dries faster than long oil varnish; it produces a harder film. It is commonly used for floors.
Short oil varnish has five to 12 gallons of oil for every 100 pounds of resins. It produces a hard film that can be hand-rubbed to a satin or gloss finish and is an ideal choice for furniture. Most manufacturers use the terms "tabletop varnish" or "rubbing varnish" to designate short oil varnish. Another term, "restoration varnish," is used for short oil varnish made from natural oils and resins. Restoration varnish is designed for finishing historic pieces.
Creating the perfect varnish finish can be a lengthy process. Open-grained woods like oak or walnut should be filled. The wood surface should be sealed, then either scraped or sanded smooth. Remove all dust and dirt with a tack cloth.
Choose a work area that is well-ventilated and dust free. Excessive humidity can cause the varnish to "bloom," that is, develop a white haze.
It is essential to use a fine-quality brush for applying the varnish. Traditionally, furniture finishers used oval brushes, because they found it easier to varnish moldings and spindles with this brush shape. If you're only working on flat surfaces, you can get by with a 21/2-inch flat brush (use a 4-inch brush for larger surfaces).
Do not work from the metal can, instead pour the varnish into a wide-mouthed jar or ceramic bowl. These vessels do not have seams and it's easy to remove excess varnish from the brush by tapping it against the sides of the jar. Do not scrape the brush across the rim; this will fill the varnish with air bubbles.
Start in the middle of the surface and brush in overlapping strokes toward the ends. Do not overload the brush; the varnish should flow evenly without runs or sags. After the surface is completely coated, "brush it out" by brushing over the entire surface - without adding any varnish to the brush - in long strokes. Some artisans "brush out" twice, first across, then with the grain.
The final technique is called "tipping off." First remove any excess varnish from the brush, then, holding it vertically, drag the brush in light strokes with the grain. The object here is to smooth the surface and remove air bubbles.
In most cases, you'll want to apply a second, third and even fourth coat of varnish. Varnish inevitably picks up dust particles. These have to be smoothed out with sandpaper before applying more varnish.
Before sanding, make sure that the varnish is completely dry and hard (this may take from four to six weeks), then sand with a 320-grit wet sandpaper lubricated with water. Remove all the sanding residue with a clean, cheesecloth pad.
For the final coat, start sanding with the 320-grit paper, then switch to 400 grit, and finish with 600-grit sandpaper lubricated with water.