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What is Varnish?

If you’re working on a project that involves wooden furniture or paneling, you’ll likely find yourself facing an important decision at the end: varnish or polyurethane for the finish? There’s a lot of confusion over the difference between these two popular wood sealants, and it’s important to know what sets them apart so you can choose the right topcoat for your project.

Varnish Home Furniture iStock

The word “varnish” is often used as a generic term for any type of wood finish, including polyurethane, lacquer, and shellac, but it technically refers to a specific combination of resins, oils, and solvents. Varnish has been around for centuries—in the beginning, the formula consisted of wood sap and alcohol—while polyurethane is a relatively modern invention made from synthetics. (Lacquer and shellac contain other ingredients and are generally used to achieve a specific look.)

What Does Varnish Do?

Varnish cures into a hard, glossy, and transparent film, although you can purchase versions tinted in various sheens and hues. Its main purpose is to seal and protect wood products while showing off the wood’s distinctive grain and natural beauty.

Varnish vs. Polyurethane

Polyurethane was created as a synthetic version of varnish. It’s basically plastic in liquid form, although in some versions it’s blended with resin. It’s available in two formats: water-based and oil-based. Water-based polyurethane dries quickly and is low in toxicity and odor, but it doesn’t always stand up well to heat or harsh chemicals. Oil-based polyurethane is more durable, but also more toxic and malodorous.

A main advantage of varnish over both types of polyurethane is that it has a higher ratio of solids, making it more resistant to water and less susceptible to ultraviolet light. That’s why varnish is typically used on wooden furniture and structures that are meant to stay outdoors and be exposed to the elements, such as lawn chairs, exterior door trim, and decks. Polyurethane, on the other hand, is normally used indoors—for example, on hardwood floors, cabinetry, and wooden furniture.

In addition, while varnish and polyurethane are both strong materials, the former is more flexible, so it can help mitigate the splitting or cracking of treated wood. Varnish is better for softwoods that flex easily, such as pine. It’s also less toxic than oil-based polyurethane.

Varnish does have its drawbacks. It can be tricky to use, as the liquid has a tendency to bubble up while being applied. For this reason, it’s not really recommended for beginners; a water-based polyurethane can be easier to apply.

Varnish is also thinner than polyurethane, so two to four coats of varnish are required to achieve the same effect as fewer coats of polyurethane. Because so many layers are necessary, varnish can take a long time to dry (at least six hours for each coat).

And if varnish isn’t allowed to dry under optimal conditions (protected from the wind and rain), it can be prone to cracking or peeling, leaving the wood underneath vulnerable to water and sun damage.

How to Apply Varnish

  1. Varnish should be applied with a natural-bristle brush, and you’ll need to take all the usual precautions for painting any surface. All dust should be removed from the wood before application.
  2. After the wood is treated, it should be allowed to dry in an area where debris and moisture can’t get to it. Remember that each coat of varnish requires at least six hours and optimal conditions to dry.
  3. Two to four coats are recommended to achieve a nice, solid finish. Allow each layer to dry completely before applying the next one.
  4. If you notice bubbles forming as you apply the varnish, try brushing the liquid on more thinly.
  5. For a completely smooth finish, use 220-grit sandpaper to lightly sand the varnish between layers.