What Is Varnish, and How Does It Compare to Polyurethane?

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. You've come across the term if you do woodworking, refinishing, or wood-centric DIY projects of any kind. But it often is used interchangeably with other types of wood coatings, including shellac, lacquer, and polyurethane. Knowing what varnish is and the difference between that and other coatings, can give you the best results on your projects.

Varnish vs. Polyurethane

Varnish is most commonly confused with polyurethane. But varnish is also different from shellac and lacquer.

• Shellac is a non-toxic finish that brings out the grain of lighter-colored wood, but it is not durable and does not protect a surface from moisture.
• Lacquer is a toxic and very durable finish that creates either a high-gloss or very matte look, unlike varnish which results in a more subdued finish.

Here are the main differences between varnish and polyurethane:

Types of Varnish

Varnish got its start as a mixture of sap (resin) and alcohol (solvent). Nowadays, many different types of varnish 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.

Spar Varnish

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 look more attractive.

Exterior Varnish

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

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

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

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.

Oil Varnish

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.

Spirit Varnish

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.

Drawbacks of Varnish

When properly applied, a varnished finish is gorgeous. But it does have its disadvantages:

• Some varnishes may not be as long-lasting as other sealants.
• Some types of varnish are best used for outdoor projects rather than interior projects.
• Some types of varnish can yellow over time.
• Beginners will find that varnish can be difficult to apply.
• More coats of varnish are required since it is thinner than other sealants.
• Varnish dries very slowly.
• A mask is required when using a varnish.

Safety Considerations

Regardless of which type of varnish you're using, you should only do so in a well-ventilated environment and while wearing a mask 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.

1. Prep Wood Surface and Work Area

   • Prep the wood surface by sanding with the grain until smooth.
   • Remove 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.

   Tip

   To remove as much dust as possible from a wood project, alternate between tack cloth wiping and blasting with compressed air. At the end, cease the blasts of air and let the dust settle before giving one last wipe with a tack cloth.

2. Thin the Varnish

   Generally speaking, the first coat of varnish goes on easier if thinned. This will help prevent bubbles and runs on the surface. To thin the varnish, do the following:

   • Pour some varnish into a clean disposable container.
   • Add mineral spirits until approximately 20 percent of the solution is composed of thinner.
   • Carefully mix until thoroughly combined.
     

   Warning

   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.

3. Apply Varnish

   • Using a natural bristle brush, apply a layer of the thinned varnish to the wood's surface.
   • Brush first against the grain.
   • Follow up by brushing with the grain.
   • Allow the varnish to dry according to the manufacturer's specifications for recoating.

4. Sand Varnish

   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.

5. Apply Second Coat of Varnish

   Now that the surface has been coated with the thinned varnish, applying the second coat will be much easier.

   • You can leave the varnish undiluted for the second coat.
   • Brush on the second coat in the same way you did the first coat.
   • Allow it to dry according to the manufacturer's requirements.

   Tip

   Always pay close attention to the manufacturer's drying times, as these are what insures the durability and longevity of the varnish finish. Sun exposure, extreme temperatures, and high or low humidity can have drastic long-term effects on a varnish finish.

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. If 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 as 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.