Polyurethane vs. Spar Urethane (What’s The Difference?)

Durability is important when it comes to choosing a finish for wood. Two of the most durable finishes are polyurethane and spar urethane. But what is the difference between polyurethane and spar urethane?

Polyurethane is a synthetic polymer gotten from a combination of chemical compounds (polyols and diisocyanates or isocyanates). Due to its strength, you can use polyurethane on exterior walls and furniture. 

On the other hand, spar urethane is gotten from the combination of several solvents, additives, and resins. You can also use spar urethane on exterior buildings and furniture. 

Spar urethane is also used to seal the hull of boats and sea vessels. This is because spar urethane is formulated with chemicals and additives to block the effects of UV lights, moisture, and harsh weather conditions.

What is Polyurethane Paint?

What is Polyurethane Paint?

Polyurethane is a plastic sealant gotten from the reaction of alcohol with isocyanates or diisocyanates compounds. Polyurethane has a synthetic or chemical make and is known for its durability. 

When dry, polyurethane produces a thick, glossy, and moisture-resistant layer. The thick layer protects the surface underneath from water, moisture, scratches, and damage. In addition, exterior polyurethane is UV-resistant.

Since polyurethane has a plastic-like formula, the finish is extra flexible and elastic. As a result, the finish of polyurethane can cope with temperature changes and heavy use because the finish is flexible enough to adapt to cold, warm, or hot temperatures.

The toughness, flexibility, and thick layer of the finish give the polyurethane its improved durability compared to other sealants.

To apply polyurethane, you can use a paintbrush, spray gun, wipe-on, or a rag. 

What Is Spar Urethane Good for?

What Is Spar Urethane Good for?

Spar urethane is a powerful and weather-resistant finish. Originally, spar urethane was formulated for the hulls (bottom) and spars (poles) of sea vessels. Spar urethane was used to protect these parts of sea vessels from saltwater, UV-rays, and crashes against rocks underwater. However, you can now use spar urethane for furniture, not just boats.

Spar urethane is made of several resins, paint chemicals, and additives. These compounds give the finish an improved resistance against weather conditions. The improved resistance and strength of spar urethane make it ideal for outdoor furniture and buildings.

Spar urethane dries by oxidization. Once the oxidization process is finished, the spar urethane produces a film (or layer) over the surface. This layer protects the surface underneath from weather elements. 

There are two types of spar urethane; water-based and oil-based. To apply spar urethane, you can use a paintbrush or spray gun.

Fun fact: Spar urethane doesn’t darken or turn yellow over time. 

Polyurethane vs Spar Utherane: Head to Head

So how does polyurethane fair when compared to spar urethane? Let’s explore both paint features to find their differences, strengths, and weaknesses.

Paint Application

How easy is it to apply both sealants?

It’s easy to apply both polyurethane and spar urethane. You can apply both sealants with a paintbrush or spray gun.

However, spar urethane tends to be more difficult to apply than polyurethane. This is because spar urethane is thicker, which means it has a high viscosity. Thicker sealants are difficult to control and tend to reveal brush marks when the sealant dries. Polyurethane isn’t as thick as spar urethane. This makes it easier to apply.

Also, you must apply more coats of spar urethane to get a sheen finish. But, you only need 2-3 coats of polyurethane. If you have a good undercoat like epoxy, you only need 1 coat of polyurethane.


Spar urethane is more UV-resistant than polyurethane. This is because spar urethane is designed with additives like UV blockers, making the paint coating resistant to UV lights.

However, only exterior polyurethane has UV blockers, while other types don’t. 

Amount of Oils

The main difference between spar urethane and polyurethane is the amount of oils the sealants have. Spar urethane has more oils and solvents, which makes it soften (and thicker) than polyurethane. 

The soft nature of spar urethane makes it more flexible for wood application (or temperature changes). Over time, wood expands due to humidity and weather, and since spar urethane is flexible, it won’t crack. 

On the other hand, polyurethane still has a lot of oils in its formula, but not as much as spar urethane has. This is why spar urethane is recommended for outdoor surfaces. 


In terms of durability, spar urethane lasts longer than polyurethane. This is because spar urethane is a fortified sealant that thrives in any environment. UV rays, moisture, dents, and all can be handled brilliantly by spar urethane. Though polyurethane is also strong, the finish will not last long in a harsh environment.

Indoor and Outdoor Use

You can use polyurethane for both indoor and outdoor surfaces. However, interior polyurethane is more suited to indoor surfaces since the sealant is not weather-resistant. Polyurethane will protect a surface from dents, scratches, and spills, but it won’t protect it from UV rays or storms. For outdoor surfaces, use exterior polyurethane. 

On the other hand, spar urethane is suited for outdoor use. The finish is formulated with additives and resins that make the sealant resistant to weather elements. As a result, Spar polyurethane will protect your furniture or wall from rainfall and dents.

You can also use spar urethane indoors, but you must use a water-based spar urethane. 

Paint Cost

Spar urethane is more expensive than polyurethane. The higher price tag is due to the impressive features of spar urethane.

Ideal Paintbrush

You can apply polyurethane and spar urethane with a paintbrush. But, you need different paintbrushes because of the difference in the flow thickness.

To apply polyurethane, use a bristled or foam paintbrush. 

To apply spar urethane, use bristled paintbrush. The finish will reveal brush marks if you use foam or a non-bristled paintbrush for spar urethane.

Paint Thinning

Before applying, you must thin (or dilute) spar urethane with a solvent. This is because spar urethane is thick, and a thick flow is harder to apply over a surface.

However, you don’t need to thin polyurethane. But, you can still thin polyurethane if you want to apply lighter coatings. 


Spar urethane is waterproof, while polyurethane is water-resistant. Spar urethane is designed for decks, hulls, and spars of sea vessels. So for surfaces that will be exposed to moisture, spar polyurethane is the best.

On the other hand, polyurethane is water-resistant. If you expose polyurethane to constant water, the polyurethane finish will blur because the water will penetrate the finish through the sides.

Dry Time

Spar urethane dries faster than polyurethane. That’s because spar urethane dries through oxidization while polyurethane dries through evaporation. 

This means the spar urethane starts to dry immediately after you apply it. It takes spar urethane 1 hour to dry to touch. 

On the other hand, polyurethane has a longer dry time. That’s because polyurethane dries through evaporation. It takes oils a lot of time to evaporate from the coating, especially in humid conditions. However, water-based polyurethane can dry within 12 hours. You can also speed up the dry time of polyurethane by using a heater, hairdryer, or heat lamp. 

Related Read: Polyurethane vs Varnish?

Deciding Between Polyurethane and Spar Urethane

Before you decide which of the two paints to buy, it’s important to answer a few questions.


Spar urethane is costlier than polyurethane. If you want a pocket-friendly option, go for oil-based polyurethane.

If you don’t mind the cost, go for water-based spar urethane. Though costlier, water-based spar urethane is more durable.

Outdoors or Indoors?

For indoor surfaces, use polyurethane. For outdoor surfaces, use spar urethane. Spar urethane is tougher and will thrive under harsh exterior conditions. 

However, other things to consider include:

Durability – Spar urethane is more durable and will protect high-traffic surfaces better. However, don’t use spar urethane on floors as the finish isn’t hard enough to coat floors. 

Your Experience – Spar urethane is more difficult to apply and best suited to experienced or professional painters. Also, polyurethane dries slower, so it’s more suited to beginners that need time while painting. Also, since spar urethane dries fast, it can be hard to get an even color and consistency as a beginner.

Final Words

Spar urethane and polyurethane are tough finishes, and picking one can be hard. However, polyurethane is better for indoor surfaces, while spar urethane is better for outdoor surfaces. 

Spar urethane dries faster, but it’s harder to apply. While polyurethane takes longer to dry, it’s easier to apply. Ultimately, your choice depends on your taste and needs.


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