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 them?

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

It 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.

Polyurethane

What is Polyurethane Paint?

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

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

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

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

Spar Urethane

What Is Spar Urethane Good for?

Spar urethane is a powerful and weather-resistant finish. Originally, it was formulated for the hulls (bottom) and spars (poles) of sea vessels. This sealant was used to protect these parts of sea vessels from saltwater, UV-rays, and crashes against rocks underwater. However, you can now use it 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. Its improved resistance and strength make it ideal for outdoor furniture and buildings.

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

There are two types; 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

It’s easy to apply both sealants, you can apply them with a paintbrush or spray gun.

However, spar urethane tends to be more difficult to apply than polyurethane because it 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.

Also, you must apply more coats 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.

Amount of Oils

The main difference between these two sealants is the amount of oils they have. Spar urethane has more oils and solvents, which makes it softer and thicker.

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

On the other hand, polyurethane still has a lot of oils in its formula.

Durability

Spar urethane lasts longer than polyurethane because it is a fortified sealant that thrives in any environment. Moisture, dents, and all can be handled brilliantly by it. 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. This sealant will protect a surface from dents, scratches, and spills.

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, it will protect your furniture or wall from rainfall and dents. You can also use it indoors.

Paint Cost

Spar urethane is more expensive than polyurethane because it has better features.

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.

Paint Thinning

Before applying, you must thin (or dilute) spar urethane with a solvent. This is because its flow is too 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 it if you want to apply lighter coatings. 

Water-Resistance

Spar urethane is waterproof. It is designed for decks, hulls, and spars of sea vessels. So, you can use it for surfaces that will be exposed to moisture.

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

Dry Time

Spar urethane dries faster because it dries through oxidation. This means it 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 because it 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?

Which One Do You Need?

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

  1. Budget – If you want a pocket-friendly option, go for oil-based polyurethane as it costs less.
  2. Outdoors or Indoors – For indoor surfaces, use polyurethane. For outdoor surfaces, use spar urethane as it is tougher and will thrive under harsh exterior conditions.
  3. Durability – Spar urethane is more durable and will protect high-traffic surfaces better. However, don’t use it on floors as the finish isn’t hard enough to coat floors.
  4. 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.

Final Words

Spar urethane and polyurethane are tough finishes, and picking one can be hard. However, polyurethane is better for indoor surfaces, dries slower, but it’s easier to apply. While spar urethane is better for outdoor surfaces., dries faster, but it’s harder to apply.

Tony Adams
Tony Adams

Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,

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