What Is Varnish?

If you have ever done any kind of home improvement with wood you will have heard of various types of wood finishes.

But if someone were to ask you what is varnish would you be able to explain it to them? Could you tell them exactly what it is and how it is used?

What Is Varnish

What Is Varnish?

Varnish is often used as a generic term for a variety of wood finishing products that include lacquer, polyurethane, and shellac.

However, varnish is a specific mixture of resin, oil and a solvent or thinner. 

Different types of varnish have different mixtures of ingredients. The purpose of varnish is to provide a fast curing, tough and hard wearing coat to wood. 

Depending on the use to which it is put varnish can provide protection, decoration and finishing to floors, doors, wooden furniture and even boats. 

Varnish Vs Polyurethane

There can be some confusion about the difference between varnish and polyurethane. While they are similar they are not the same product. 

Varnish has been around for centuries and consists of resin, oil and solvent. Polyurethane was created as a synthetic version of varnish.

It comes in two forms, water based and oil based. In its simplest form polyurethane is a type of liquid plastic. 

In terms of durability varnish is tougher and more resistant to UV light than polyurethane. It is also more resistant to water such as yacht or spar varnish. 

How To Apply Varnish

A natural bristle brush should be used to apply varnish and the wood surface should be well prepared before applying.

After application the varnish should be allowed to dry in an environment away from dust, debris and moisture.

Two or three coats of varnish are typically applied and the time between coats is typically around 6 hours, but this depends on the ambient conditions. 

However long it takes, each coat should be dry before the next one is applied, lightly sand the surface between applications. 

If bubbles start to appear as you apply the varnish, apply it more thinly. 

Types Of Varnish

Types Of Varnish

There are many different types of varnish, let’s take a look at some of them. 

Spar Varnish

Spar varnish is named after the wooden support of the sail on a yacht or ship. This type of varnish was typically used to protect this and other wooden parts of sailing ships and boats from the elements. 

Originally spar varnish was designed to be flexible as the sail of a boat would cause the mast to bend. But although it was flexible it wasn’t very resistant to UV light. 

Modern spar varnish is highly durable, UV resistant and made to decorate as well as protect parts of a ship. 

Exterior Varnish

Developed from spar varnish, exterior varnish is specifically designed to be used outside. This finish is used to protect wood that is exposed to the elements including sun, rain, wind, and frost.

Often used on outdoor furniture, decking, or exterior doors, exterior varnish gives a hard wearing coating that preserves your wood for longer.

Some makes are fortified with UV absorbers and chemicals to inhibit mildew and mold growth on your exterior wood. 

Good quality exterior varnishes do not go yellow over time, so your wood will preserve its natural color. 

Alkyd Varnish

Alkyd varnishes are highly versatile as they can be used both indoors and outdoors.

They are made from polyester resins which have been modified with fatty acids, often extracted from vegetable oils. These are used to speed up the curing process.

They have the benefit of being fast drying while still being durable.

Many alkyd varnishes offer clear coat protection with the addition of enhanced UV resistance. This is particularly important for outdoor applications or on sun exposed rooms. 

Alkyds also have good moisture and solvent resistance. 

Bituminous Varnish

A bituminous varnish is one where the resins have been replaced with bitumen which is a petroleum based hydrocarbon.

Bituminous varnish is also referred to as black varnish because of this ingredient which acts as a hardener in the absence of a resin.

It has excellent adhesion to most surfaces and is highly waterproof.

Most commonly used as a protective coating for ironwork, stoves and marine accessories the bituminous varnish forms a protective barrier against the elements. 

There are many types of black varnish used for different purposes. 

Acrylic Varnish

Acrylic varnish is water based and as such is a quick drying finish for wood. It is highly transparent and resistant to yellowing unlike some oil based varnishes.

This varnish is used as a protective coat over the top of paintings to preserve them. 

On wood, it does not penetrate as well as oil based varnish but also does not have the harsh, solvent fumes.

However, it does have good UV resistance. Clean up after using acrylic varnish is a lot easier as brushes can be washed in a water solution. 

Oil Varnish

Oil varnish is the combination of a resin, either natural or synthetic with a drying oil such as linseed or tung oil. There is no solvent in oil varnish. 

The drying oil in oil varnish dries to a solid when exposed to oxygen while the resin in the varnish contributes to the durability of the finish. 


Shellac is a single component resin varnish that is widely used for interior applications. It is unsuitable for exterior use and should not be exposed to contact with water. 

For this reason it is mainly used as a decorative varnish for fine furniture. It is the basis of French polishing and is processed from the secretions of an Asian insect. 

Shellac is typically available in a clear or amber variety. Cloths and brushes must be cleaned with pure alcohol or ammonia. 

Final Thoughts

There are many different types of varnish, interior, exterior, oil based and water based. Choosing the right one depends on what it will be applied to and its intended use. 

We hope you found this guide to varnish helpful. 

Joel Adams
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