If you’ve dabbled in woodworking, refinishing, or wood-centric DIY projects of any kind, you’ve likely heard of a type of sealant coating called varnish. While many have heard of varnish, few actually know what it is. This isn’t because varnish is some mysterious product that is hard to understand, but rather because the term is tossed around quite loosely as an umbrella for many different types of wood sealants, sometimes accurately but oftentimes inaccurately.
For years, the word "varnish" has been used as a generalized term to describe many kinds of wood coatings, including shellac, lacquer, and polyurethane. However, though there are many different types, varnish is a specific product rather than a generic term or grouping of other types of products.
What Is Varnish?
Varnish is a centuries-old, slow-drying sealant composed of specific resins, oils, and solvents that creates a translucent, highly protective coating when applied to wood.
Varnish vs. Polyurethane
Started as mixture of natural ingredients
Trickier to apply
Contains more solids
Bubbles more during application
More flexible finish
Very long drying time between coats
Thinner and requires more coats
Started as synthetic alternative to varnish
Easier to apply
Contains fewer solids
More rigid finish
Relatively short drying times
Thicker and requires fewer coats
Water or oil-based
Types of Varnish
Varnish got its start as a mixture of sap (resin) and alcohol (solvent). Nowadays, there are many different types of varnish that have been created to serve a variety of different purposes. This process of creating new varnishes is simply done by manipulating the ingredients that go into a varnish—resin, solvent, and oil—as well as the ratio of the ingredients present in the mixture.
Listed here are the most common types of varnish, as well as what distinguishes them from one another.
A spar is a strong rod, typically made of wood, that supports the sail on a ship. Spar varnish, or yacht varnish, is a traditional type of varnish used to protect the spar and other wooden parts of a boat or ship that are highly prone to damage from the elements. Because of its intended use, spar varnish was, historically, very flexible but not very durable. However, modern versions of spar varnish have been altered to be more durable, more UV-resistant, and more attractive in appearance.
Exterior varnish is an example of modernized, modified spar varnish. Sometimes labeled as spar urethane, it differs from true varnish. Exterior varnish possesses the protective, flexible characteristics of spar varnish, but has been modified to be UV-protectant and more durable, thanks to its slower curing time.
Alkyd varnish is a highly versatile varnish that can be used both indoors and outdoors. The word "alkyd" refers to the alkyd resins in the varnish, which are polyester resins modified with fatty acids commonly derived from vegetable oil. Many alkyd varnishes offer great UV protection, quick-drying, yet durable finishes, and a clear appearance that greatly enhances the beauty of the wood.
Bituminous varnish, or black varnish, refers to any type of varnish in which the resins are replaced with bitumen (petroleum-based hydrocarbon). The main purpose of black varnish is to coat ironwork, protecting it from corrosion.
Acrylic varnish is a water-based protective coating that is far from traditional varnish in its makeup. Because it's highly transparent, easy to apply, and anti-yellowing, the most common use of acrylic varnish is as a protective coating applied over the top of paintings.
Though it's debatable whether or not oil varnish is a true varnish, it is commonly referred to as such. Oil varnish is simply a mixture of resin and drying oil, absent of a solvent. This results in a finish that penetrates deeply into the wood and hardens over a very long period of time.
Similarly to oil varnish, it's debatable whether or not spirit varnish is a true varnish. While oil varnish is a mixture of resin and drying oil, absent of a solvent, spirit varnish is a mixture of resin and a solvent, absent of oil. Spirit varnish is quick-drying and highly polishable, but not nearly as durable as oil varnish.
Regardless of which type of varnish you're using, you should only do so in a well-ventilated environment and while wearing a respirator rated for fumes. Not all varnishes are created equally, and before you choose one and use it on your project, pay special attention to its intended uses. For instance, using varnish with a weeks-long cure time that releases a high amount of VOCs (volatile organic compounds) in a space that you or anyone else frequents will pose great risks to the health of those exposed.
How to Use Varnish
The following steps will outline the process of varnishing a flat wood surface using alkyd varnish. These steps will vary depending on the item you wish to varnish and the type of varnish you choose.
Prep Wood Surface and Work Area
Prep the wood surface by sanding with the grain until smooth and removing all dust with a tack cloth. Additionally, remove all dust from your work area to ensure no dust, dirt, or debris ends up in your wet varnish.
Thin the Varnish
Generally speaking, the first coat of varnish goes on easier if thinned. This will help prevent bubbles and runs in the surface. To thin the varnish, pour some varnish into a clean disposable container, then add mineral spirits until approximately 20 percent of the solution is composed of thinner. Carefully mix until thoroughly combined.
Mineral spirits will not work as a thinner with every type of varnish. Consult with the manufacturer's instructions on your particular type of varnish for guidelines regarding thinning, or you may risk damaging or ruining the surface you are varnishing.
Using a natural bristle brush, apply a layer of the thinned varnish to the wood's surface. Brush first against the grain, then follow up by brushing with the grain. Allow the varnish to dry according to the manufacturer's specifications for recoating.
Once the varnish has dried long enough for a second coat (typically around 18 hours under perfect drying conditions), sand it using 220-grit sandpaper, then thoroughly remove all dust.
Apply Second Coat of Varnish
Now that the surface has been coated with the thinned varnish, applying the second coat will be much easier. This means you can likely leave the varnish undiluted. Brush on the second coat in the same way you did the first, then allow it to dry according to the manufacturer's requirements.
How to Care for Varnished Wood
Maintaining varnished wood surfaces and keeping them free of debris generally only requires an occasional wipe down with a dry, soft, lint-free cloth. In the event that there is stubborn, grimy residue, using a lightly damp cloth to free the residue may be necessary and should be immediately followed up with a dry, lint-free cloth. To bring out the shine of a varnish surface, a furniture polish that is specified safe for varnish will do the trick.
When to Replace Varnish
If the varnish on a piece of furniture or other wooden item is looking worse for the wear, it may be time to strip it and replace it. While some people advise adding new varnish overtop of existing varnish, this likely isn't a good idea in most cases. It is better to strip the varnish using a stripping gel or sandpaper, or a combination of the two, then prep the surface and apply new varnish.
"Toluene Safety in the Workplace." OSHA Infosheet, United States Department of Labor Occupational Safety and Health Administration.