Spar Urethrane VS Polyurethrane: What’s Right for You?

spar urethrane vs polyurethrane

When it’s about time to finish your woodwork project, there is often the dilemma of choosing between Spar Urethane and Polyurethane. Don’t get me wrong. That is a choice that all woodturners have had to make each time they come up with a furniture or wood project. But in the past, the process of making that choice between Spar Urethane and Polyurethane was easy. There were living breathing guides, and products were not so many. Not forgetting the sheer number of conflicting information out there about these two varnish products.

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Sadly, today, we are missing professional guides, and misleading information is not doing us any good. That is why you have people like me to help.

Like the making of any excellent woodwork, choosing between Spar Urethane and Polyurethane is not a pretty sight. There is plenty of information out there on both products. Compiling them to find what is best is often a complex task. Many woodworkers spend years accumulating books and magazines to back their use of these two. In my post today, I want you to know the similarities and differences when it comes to Spar Urethane and Polyurethane

Compare Spar Urethane vs. Polyurethane

Today, the term spar urethane refers to a mix of spar varnish and urethane. The urethane can work with water, and this is why this varnish is suitable for outdoor use. You may have already come across the Minwax Helmsman brand as a type of spar urethane. Often, it next shelf to a Cabot Marine Spar Varnish, which is a spar varnish without the urethane.

On the other hand, Polyurethane is a plastic resin varnish that hardens after application. The scientific side of things also explains it as a single urethane compound that forms after the process of polymerization. You will come across water or oil-based polyurethane varnishes when deciding on this type of varnishes. But there are other types of polyurethane varnishes coming up but with all the similar water-aspect as the Spar Urethane.

Now, this means Spar Urethane and Polyurethane are similar coatings but with a slight difference. Not really. Usually, the comparison does not end with the ingredients alone. In this next part, we interchangeably compare these two vanishes to see their similarities and differences across various aspects.

  1. Properties
  2.     2. Application
  3.     3. Toxicity
  4.     4. Pros
  5.     5. Cons

           ▪ Properties

Polyurethane is a liquid plastic coating that works as an oil or water-based finish with relative ease. The oil-based polyurethane varnishes are the most popular you will come across as the best seller in their line. The selling point is that they come with plenty of advantages that we will get to explore later on. But the water-based varnishes are also popular in their own right with the Minwax Polycryclic, an example of a popular water-based polyurethane.

Further, you will also find that exist different types of polyurethane products for a variety of uses. And there is still advancement in using the polyurethane compound to come up with a broad spectrum of materials thanks to its make. Apparently, due to its diisocyanates and polyols being many, the mixture can produce a wide range of vanishes to meet specific applications.

On the flip side, there is the spar urethane. Now, it is also a urethane that comes as a clear protective varnish for both interior and exterior woodwork. Another character is that it does not yellow even after exposure to weather, changes in temperature, or sunlight. Like the specification, finishes in this range come with UV blockers and water-resistant capabilities that are crucial to give it the advantages.

            ▪ Application

The majority of the difference in Spar Urethane and Polyurethane comes when it is time for implementation. While both vanishes in use can be oil-based products, there is the chance that application may differ.

For example, you don’t need to use Spar urethane any differently from how you use typical varnish. You will only need a brush, clean wood surface, and ambient temperature. In most circumstances, 50 degrees Fahrenheit is the recommendation for the spar urethane to work after application. Since it comes with a thick consistency, it will take longer to dry before you are set to apply another coat. Usually, it would take 4-6 hours to dry out before second and after layers are possible within a sitting of 24 hours.

And while re-applying, it wise to ensure that there are no visible brush strokes due to brushing against the wood grain. To save you the time and effort here, I would suggest the Presa Brush toolkit since it offers variety during application. The different brushes can save on time and effort when applying each fresh coat.

When it comes to applying polyurethane vanishes, the situation is much different. First, it’s most delicate when it comes to the application since its prone to imperfections. From my experience, the finish often shows bubbles and brush strokes when the use is not perfect. That is due to polyurethane being a thicker solution than the spar polyurethane.

The suggestion is that a thinning solution with up to 50 percent of mineral spirits would help avoid the imperfections. Changing the brush in use would also help prevent the evidence of brush strokes. There is a range of particularly fine brushes for polyurethane available to use in case you want the perfect results.

While applying polyurethane with brushes is the best, it’s not the easiest. As you are aware, the vanish solution is thick and will still need more coats. You can save yourself the effort and time by using the wiping method on your furniture or woodwork surface. It will only take you a spray, paint sponge or roller to get that shiny finish after application.

            ▪ Toxicity

There is also debate on which between Spar Urethane and Polyurethane is most appropriate when it comes to toxicity levels.

Polyurethane is the most toxic of the two since its a plastic resin. But, water-based and newer polyurethane varnishes are non-toxic blends that you can use safely. Another water-based polyurethane is slightly toxic but still safe for use.

The oil-based polyurethane is a toxic solution that will need you to take precautions and safety measures to avoid poisoning. That would require you to use the vanish solution in a well-ventilated room or well outside. For extra safety, you may need protective clothing and gear such as respirators and gloves.

But for Spar Urethane, toxicity is the opposite. The urethane finish is less toxic, especially before undergoing dilution. You are aware that it will need a 20 to 40 percent thinning with mineral spirit at first. While it doesn’t make it toxic, precaution is necessary during mixing. The thinning also helps give different results from the regular application. In short, it is a way to avoid the brush strokes and bubbles from appearing with each coat application.

            ▪ Pros

You would not consider Spar Urethane and Polyurethane without thinking of the advantages. Both vanish solutions pit each other for which has the most benefits over the other.

For starters, Spar Urethane is a consideration for the first place for several reasons. You already know its less toxic than Polyurethane in regards to the application. In short, this means there are fewer or no strong fumes or odors when applying the varnish on your furniture or woodwork.

Another quality with the urethane varnish is the UV blockers. Fro the product to prevent wear, tear, yellowing, and fading of wood surfaces, this urethane offers the blockers to reduce sun damage. Such a capability makes it a handy choice for the durability of wood surfaces susceptible to such weather conditions.

You can count on Spar Urethane to further provide water-resistant characteristics. Already the wood finish is popular in use for boat finishing due to the protective nature of the urethane-based film. That alone makes it a preference for use for surfaces exposed to water. You can consider it an excellent choice for both outdoor and indoor use.

Likewise, Polyurethane also has its own set of pros when it comes to its use. The first aspect to consider is that its a thicker solution in comparison to Spar Urethane. From the outlook, this may be due to the oil percentage found in the finish. Since its more than what is present in water-based Spar Urethane, you can expect to much smoother, and glossy-like finish on your furniture.

The thick finish also means Polyurethane vanishes can resist harsh environmental conditions. And much like the Spar Urethane vanishes, it is also able to resist oxidation to ensure your furniture lasts longer.

While they are toxic, polyurethane vanishes also count on having a low toxicity level. The water-based polyurethanes and other newer vanishes are less toxic compared to their oil-based counterparts.

Another significant gain with polyurethane also has to be its flexibility in use. Earlier, I did mention that it’s available in a variety of vanishes that suit different purposes. That makes it an all-in-one vanish for use across various industries. You will see its application across the apparel, automotive, construction, electronics, and marine industries.

The last bit of advantage lies in the application of Polyurethane. Despite having a thick consistency, you still get the option of choosing between a brush, sponge, roller, or spray. That is unlike the Spar Urethane, where application and the best results come through using regular brushes during coating.

            ▪ Cons

The cons of Spar Urethane and Polyurethane further vary to an extent. One of the significant drawbacks to starting with are behind the Polyurethane finishes. With the toxic levels present, it would be a hazard to use it for your furniture without precautions. Too much exposure to the poisonous fumes can cause headaches, dizziness, and even difficulty in breathing.

Another disadvantage with Polyurethane is the intense application. The oil-based versions especially will need a delicate use to avoid open brush strokes and air bubbles from appearing. Even on the surface, expert woodworkers would still admit it needs plenty of attention.

You can also regard the compound as a very resistant solution to clean. Once the application is complete and dry, re-touching, refurbishing, or washing the polyurethane finish becomes a task. The vanish being scratch-resistant only makes it further tricky when it comes to removing it after application.

Spar Urethane also has its fair share of disadvantages. For starters, it is a water-based solution, meaning mixing will be a problem. That is especially the case if you are planning to use it on oil-based products such as paint. While there will be no significant damage, it will cause discoloration or bring unbalance in color.

Additionally, you will have to consider that Spar Urethane and Polyurethane are both toxic solutions. Polyurethane remains the most toxic of the two, but Spar Urethane also has the same characteristics. While the toxicity levels are not the same, they are still evident. That means using Spar Urethane will need adequate precautions for ventilation. Be sure to, thus, use it in a room with proper ventilation.

As an extra safety measure, consider also having a fire extinguisher during application. Both Urethanes are highly flammable compounds meaning they can easily catch fire at your workshop. You can, however, stop this in advance by having the necessary measures ready for use in case of emergency.


As far as varnishes go, Spar Urethane and Polyurethane are the most that receive attention. You can tell from our post that they are almost similar compounds with only a bit of twist. Hopefully, this has been educative enough from your end. For most of the part, the information could help you judge which varnish works where best.

But for me, Spar Urethane is a better choice for outdoor and wet weather conditions. Some of my woodworker buddies consider the Polyurethane better for indoor applications. Whatever the perspective, both these varnishes consider the essential elements of weather resistance, flexibility, and application in their use. You could say both Spar Urethane and Polyurethane makes an excellent choice depending on your woodwork project.

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