Varnish is made by cooking a drying oil with a resin, producing an amber-colored finish that is extremely durable and protective. In fact, varnish is the most durable finish that can be easily applied by the average woodworker. Although not as easy to apply as oil/varnish finishes, varnish is much more resistant to water, water vapor, chemicals, and heat. The characteristics of a varnish are determined by the type of resin, type of oil, and the proportion of resin to oil. The oils used in varnish include linseed oil, tung oil, soybean oil, walnut oil, and safflower oil. Originally, the resins used in varnish came from fossilized pine tree sap. Nowadays, most varnishes use a synthetic resin, usually alkyd, phenolic, or polyurethane.
Alkyd is the most common type of varnish for interior applications. Although not as tough as phenolic varnish, it is cheaper and doesn’t yellow quite as much. If the label doesn’t specify the type of resin in a varnish, chances are it is made from alkyd. Many “polyurethane” varnishes sold today are actually an alkyd varnish modified with polyurethane resin – a so-called uralkyd.
Phenolic varnish was one of the first synthetic varnishes. It is typically formulated with tung oil, UV inhibitors, and a high percentage of oil to form a flexible, moisture resistant finish for exterior applications. Spar varnish is an example. Phenolic varnish cures tough yet flexible. It also tends to yellow faster than other varnishes.
Polyurethane varnish (also called urethane) provides the most resistance to scratches, heat, and solvents of all the varnishes. However, it has a somewhat cloudy appearance and tends to peel when exposed to direct sunlight. It is most appropriate for indoor applications.
Many of the finishes that you see advertised as “oil” finishes are actually wiping varnish; that is, varnish that has been thinned with mineral spirits. These finishes are popular because they are relatively easy to apply – just wipe on/wipe off. However, it will take many more coats of wiping varnish to attain the same level of protection as full strength varnish because the wiping process leaves such a thin resin film. On the plus side, wiping varnish is less problematic in terms of brush marks, sags, and dust nibs.
- Excellent resistance to abrasion, wear, heat, solvents, water and water vapor.
- Relatively easy to apply with a brush (doesn’t require spray equipment).
- Inexpensive and readily available.
- Cures very slowly allowing dust to settle on the surface of the finish.
- Yellows over time. The clearest varnishes contain alkyd resin with soybean or safflower oil.
- Difficult to repair or remove (due to its exceptional solvent resistance).
- Can be difficult to rub out compared to finishes like lacquer.
Behlen Rockhard Table Top Varnish
Cabot Oil Based Polyurethane Varnish
Deft Interior Polyurethane
Epifanes Clear Gloss Traditional Varnish
General Finishes EF High Performance Polyurethane
Interlux Goldspar Satin Varnish
Interlux Goldspar Clear Polyurethane
Interlux Schooner Varnish
McCloskey Heirloom Varnish
McCloskey Gym Seal
McCloskey Spar Varnish
Minwax Fast-Drying Polyurethane Varnish
Minwax Polyshades One Step Stain & Polyurethane
Minwax High-Build Polyurethane
Minwax Helmsman Spar Varnish
SYSTEM THREE Urethane Spar Varnish
Z-Spar Captain’s Varnish
Z-Spar Polyurethane Clear Varnish
Z-Spar Satin Sheen Varnish
Z-Spar Flagship Varnish
Varnish is typically applied with a brush. (It is reasonably easy to spray but tends to create a sticky mess when uncured particles in the air settle out). Brush on the varnish in any direction as long as the final coat follows the grain. Use long, steady strokes. Remove any excess finish by tipping off the brush (wiping the brush over the lip of a clean container). Allow to cure overnight before applying the next coat, making sure you sand lightly between coats. Generally, two to three coats will suffice.
A dust-free environment is essential when using varnish because it cures slowly, allowing lots of time for dust to settle onto the surface. Wet-mop the floor in your finishing area beforehand. Wipe down nearby surfaces with a damp cloth. Also wipe the wood’s surface with a tack cloth prior to applying the varnish.
The ratio of resin to oil in a varnish makes a difference. Varnishes that contain a high percentage of oil are called long-oil varnishes. Such varnishes are ideal for outdoor applications because they are more flexible than shorter-oil varnishes and thus more forgiving of seasonal wood movement. Examples include marine, spar and other exterior varnishes. Trivia: The name “spar varnish” comes from varnish that was first used on wooden masts of ships, called spars.