Polyurethane vs Varnish: Which Is The Better Finish For Woodworking?

We all want our woodworking projects to turn out well and seek a professional appearance when completed.  Accurate cuts, solid assembly, and stain or paint all play an important part in that process, but the final touch is the finish. 

When it comes to finishes, there are many choices, from polyurethane to varnish, to shellac, to wax, and to oil, among many.

Today we want to compare and contrast two of them:  polyurethane and varnish.  It’s like we’ve all used at least one of them, if not both, and have probably developed a preference.  Yet each of them has much to contribute to the finished product (pun intended) and much to commend to their use depending on where and what the project is  – interior or exterior.

What is Polyurethane?

Polyurethane’s development dates back almost 100 years, when it was used in a number of ways, including making shoe soles, mattresses, and foam insulation. It was and continues to be used as a wood finish, as well.

We know it is non-toxic, food safe, and with the right number of coats, will create a hard and durable protective surface on the wood. It is a common choice today for a wood finish and has replaced varnish as the most common. It provides a smooth finish when applied correctly and a protective finish with the right number of coats.

It is composed of two main substances:

Polymers

Polymers can be a natural or synthetic substance that is composed of large molecules, and although that may not make much sense to you, examples will: in the synthetic category, we find nylon, Teflon, polyethylene, and epoxy; and in the natural category, we find silk, wool, protein, and DNA.

Now, we’re not giving our woodworking project a coat of Teflon, silk, or wool, but we are giving it a coating of something that will protect the wood, just as Teflon protects pots and pans. Viewing polymers as a form of plastic, it means we are coating our project with a tough, hard, and durable coat of plastic to protect it from moisture and heat.

Urethanes

We’ll keep this one a little simpler and more straightforward. Urethanes are near to the family of plastics, but more so in the family of rubber. Polymerization occurs when the two substances are combined, and that process is what gives polyurethane protective properties when applied on the wood surface.

The polymers (poly) and urethane together make polyurethane.

Polyurethane History

Otto Bayer is credited with the development of polyurethane in 1937 while working at IG Farben. Among its original uses was as a coating on airplanes during WWII.

As more uses for poly were developed, its composition was adapted for them, and it became a popular and ubiquitous commercial product in the industry. Poly foams were developed, and harder forms of poly were made available for the making of furniture and clothing.

Eventually, its use as a protective coating for airplanes led to its use as a protective coating for wood in general, and today we know it in the woodworking shop as a popular and effective topcoat for our woodworking projects. It’s coating on the wood surface hardens and offers effective protection from moisture and water spills and, to a degree (pun intended), from heat.

Types of Polyurethane

The mixture of polymers and urethanes to form the polyurethane we know today needs a medium to hold it, and when it comes to polyurethanes, there are two types that we all know and use in our work. Each has its own advantages and disadvantages that depend on the type of wood being used and the use to which the project will be put.

Water-based Polyurethanes

The first type is water-based polyurethane. As you would expect, it uses water as the medium to hold it together and carry the poly solids and is probably the one most commonly chosen by the woodworking hobbyist for simple projects around the house.

It dries quickly to a hard and durable finish but requires multiple coats to offer the type of protection we all want for our projects. While more coats can equal greater cost, there have been advances in the composition formulae for water-based polyurethane that can reduce the number of coats for the best protection.

However, we will say that more coats of water-based poly do make it more effective in protecting against water, mold, and fungus.

Oil-Based Polyurethanes

The second type is oil-based polyurethane which uses, as you may expect, oil as the medium to carry the poly solids. In the past, oil-based polyurethane was considered more durable than water-based poly, but advancements in formulas have put them mostly on equal footing.

The use of oil-based polyurethane requires more safety measures to be followed than does the use of water-based poly. The VOCs (volatile organic compounds) found in oil-based poly are toxic until it has dried fully (24 – 48 hours needed) and cured (30 days needed). Overexposure to them is dangerous and can lead to respiratory problems and lung damage.  

A respirator should always be worn when using them, and the work environment should be very a well-ventilated area. Even then, eye, nose, and throat irritation may result.

While many woodworkers swear by oil-based poly, especially for wood floors, most find them too inconvenient to work with, not only because of the necessary safety precautions but also because of the extended drying and curing time. They also tend to darken over time and are more at risk of denting than a water-based poly.

Applying Polyurethanes To Your Project

There are choices for the method of application when it comes to polyurethanes, and it’s really your preference, as each method will get the job done.

• Brush. While originally, it could be applied only by natural-bristle brush, advancements in formulas do make other means possible. But a brush still works, whether it’s a synthetic brush or a natural-bristle brush, although it may present problems. It is possible that bubbles will form during application that will require sanding between coats. While sanding is certainly advised, it is absolutely necessary when bubbles do form. A foam brush will mitigate against that and also will not leave brush marks.
• Rag. Wipe-on is another application method for poly. Less chance of bubbles and certainly no brush marks, and the application will be much smoother. There are even some poly products that are made specifically for a rub-on application, like Minwax Wipe-On Polyurethane, both oil-based and water-based.
• Spray. This is our preferred method of applying poly. We get greater control over the evenness of application, no bubbles or brush marks, and a smoother coating on the wood surface.  Polyurethane should be thinned for spraying, and you should plan on multiple coats.

While there are some products that claim to be one-coat polys, they are 3-times as thick and more difficult to apply than either water-based poly or oil-based poly. As a consequence, we’ve never used them.

Maybe out of curiosity, someday we will see if they live up to the billing, but that won’t happen any time soon. For now, we are satisfied with 3-5 coats of polyurethane for our projects when we use a poly, and for us, we prefer a water-based polyurethane.

What Is Varnish?

Varnish is a clear, hard, protective finish or film primarily used in wood finishing but also for other materials. It is traditionally a combination of a drying oil, a resin, and a thinner or solvent, plus a metal drier to accelerate the drying.

However, different types of varnish have different components. After being applied, the film-forming substances in varnishes either harden directly as soon as the solvent has fully evaporated or harden after evaporation of the solvent through curing processes, primarily chemical reaction between oils and oxygen from the air (autoxidation) and chemical reactions between components of the varnish.

Resin varnishes dry by evaporation of the solvent and hardens quickly on drying.

Varnish is used to protect wood from moisture, scratches, and fading. It also enhances the natural beauty of the wood. Varnish is available in a variety of finishes, including gloss, semi-gloss, and satin. It can be applied with a brush, roller, or sprayer.

Varnish is a relatively easy finish to apply, but it is important to follow the manufacturer’s instructions carefully. Varnish should be applied in thin coats, and each coat should be allowed to dry completely before applying the next coat. It is also important to sand between coats to create a smooth finish.

Varnish should be reapplied every few years to maintain its protective properties.

Marine varnish is used on boats, as you might expect, a clear, hard, protective finish that is used to protect wood from the harsh marine environment. It is made with a special formula that includes ultraviolet (UV) inhibitors to protect the wood from the sun’s harmful rays. Marine varnish is also resistant to water, salt, and mildew.

Marine varnish should be reapplied every few years to maintain its protective properties, and here are some of the benefits of using it:

• It protects wood from the harsh marine environment.
• It is resistant to water, salt, and mildew.
• It is available in a variety of finishes.
• It is relatively easy to apply.
• It provides a long-lasting finish.

Spar varnish is a type of varnish that is specifically designed to be used outdoors. It is made with a special formula that makes it more resistant to water, sunlight, and other harsh elements. Spar varnish is available in both oil-based and water-based formulations.

Spar varnish is a clear, hard finish that is used to protect wood from the elements. It is also used to enhance the natural beauty of the wood. Spar varnish is available in a variety of finishes, including gloss, semi-gloss, and satin. It can be applied with a brush, roller, or sprayer.

• It protects wood from the elements.
• It is resistant to water, sunlight, and other harsh elements.
• It is available in a variety of finishes.
• It is relatively easy to apply.
• It provides a long-lasting finish.

So, within the varnish family are several types, each with a specific advantage and strength when used for its intended purpose – marine environment and general outdoor environment.

Comparison and Contrast of Polyurethane and Varnish As A Wood Finish

 We now know what they are and have an idea of their differences.  Let’s recap and then compare their strengths and weaknesses to help us decide where, when, and on what to use each.

Varnish and polyurethane are both clear finishes that are used to protect and enhance the appearance of wood. They are both available in oil-based and water-based formulations.

Varnish, the more traditional wood finish, has been in use for centuries as a finish. It’s made from a resin that is dissolved in a solvent, such as turpentine or mineral spirits. Varnish dries to form a hard, protective film that is resistant to water, scratches, and fading.

Polyurethane is a newer type of wood finish that is made from a synthetic resin. It is available in both oil-based and water-based formulations. Polyurethane dries to form a hard, durable finish that is more resistant to water and chemicals than varnish.

Which is better?

The best type of finish for your project will depend on the following factors:

• The type of wood you are finishing. Varnish is a good choice for softwoods, while polyurethane is a better choice for hardwoods.
• The location of the finished piece. Varnish is a good choice for indoor projects, while polyurethane is sometimes a better choice for outdoor projects.  However, varnish provides much better UV protection because of the solids it contains.  Ultraviolet light is harmful to wood over time, and varnish can extend that time to offer greater protection than can polys.
• Your personal preferences. Some people prefer the natural look of varnish, while others prefer the added durability of polyurethane.

Varnish

• Pros:
  • Easy to apply
  • Affordable
  • Available in a variety of finishes
  • Gives wood a natural look
  • Better protection against sun damage
  • Less toxic than polyurethane
• Cons:
  • Not as durable as polyurethane
  • Not as resistant to water and chemicals
  • Can yellow over time

Polyurethane

• Pros:
  • More durable than varnish
  • More resistant to water and chemicals
  • Does not yellow over time
• Cons:
  • More difficult to apply
  • More expensive
  • Not as available in a variety of finishes
  • Does not protect against UV rays
  • Can give wood a slightly plastic look

We discussed both polyurethane and varnishes together in an earlier article on good finishes for wooden coasters, so check that piece out for more information about them.

Here are 2 that we recommend:

A Video on Varnish

We’ve used plenty of videos on polyurethane but never used a video description devoted just to varnish, so today, we chose one for you.  It actually is pretty interesting, and we recommend you take 6 minutes to watch it.  We guarantee you will learn something new.

Each has its own strengths to commend it to you as a topcoat for your project, and each has its own drawbacks.  But, each should be in the discussion, as your project will benefit from either.  One is better for outdoor use, while each is a good and safe choice for indoor use.  The other major difference has to do with toxicity.

Beyond that, it’s a matter of personal preference, with varnish getting the better nod from us in terms of appearance.Polyurethane

Last update on 2024-09-09 / Affiliate links / Images from Amazon Product Advertising API