Polyurethane vs. Spar Urethane (Are They The Same?)

Durability is very 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 vs spar urethane? Here is the answer to that.

Polyurethane is a synthetic polymer gotten from a combination of chemical compounds (polyols and diisocyanates or isocyanates). Polyurethane finish is commonly used on interior walls and furniture due to the strength, durability, and moisture-resistance of the finish.

Spar urethane on the other hand is gotten from the combination of several solvents, additives, and resins. It is commonly used on building exteriors and outdoor 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 common for its durability. Polyurethane has a synthetic or chemical make-up. The finish is gotten from the reaction of alcohol with isocyanates or diisocyanates compounds.

Polyurethane when dry produces a thick and moisture resistant film on wood or material. The thick film is sometimes formulated with additives to give it UV-resistant qualities. However, this is for exterior polyurethane.

Polyurethane is also flexible. The plastic-like nature of the finish gives it extra flexibility and elasticity. Polyurethane can cope with temperature changes and heavy use as the finish is flexible enough to adapt to cold, warm, or hot temperatures.

The toughness and flexibility of the finish are what give it improved durability when compared to other sealants. Polyurethane can be applied with a paintbrush, spray gun, or wipe on.

Now, let’s check out what spar urethane is used for.

What Is Spar Urethane Good for?

What Is Spar Urethane Good for?

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

Spar urethane is made of several resins, paint chemicals, and additives. All of these compounds are included in the paint make-up to give it improved resistance against weather conditions. The improved resistance and strength of spar urethane make it ideal for outdoor furniture and building exteriors.

Spar urethane dries by curing. The paint goes through oxidization to produce a film on the material or paint used. Like many other stains, spar urethane is either water-based or oil-based. Spar urethane can be applied with a paintbrush or spray gun and unlike several sealants, spar urethane doesn’t darken or turn yellow over time.

However, spar polyurethane has a few flaws. The paint is flammable.

Polyurethane vs Spar Utherane: Head to Head

So how does polyurethane fair when compared to spar urethane? Let’s go deeper into both paint features to find out their differences, strengths, and weaknesses. First, let’s start with the…

Paint Application

How easy is it to apply both paints?

It is quite easy to apply polyurethane and spar urethane. You can also apply both paints using the same methods. 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 paints are difficult to control and tend to reveal brush marks when the paint dries.

Polyurethane however is not as thick as spar urethane. This makes it easier to apply polyurethane. Also, you’ll need more coats of spar urethane to give the sheen and desired result.

Polyurethane on the other hand needs just 2 or 3 coats. If you have a good undercoat like epoxy, you can get the desired finish with just one coat of polyurethane.

UV-Resistance

How do both paints fair under sunlight?

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

Polyurethane also has UV blockers but not in every polyurethane product. UV-blockers are more common in exterior polyurethane.

Amount of Oils

One of the most common differences is the amount of oils the paints have. Spar urethane has more oils and solvents which make the paint coating softer than polyurethane.

That makes spar urethane more flexible when it comes to wood application. Wood over time expands due to humidity and weather, and spar urethane is less likely to crack.

Polyurethane still has a lot of oils in the mix, but not as much as spar urethane. That’s why spar urethane is recommended for outdoor applications.

Durability

How long do the paints last?

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.

Heat-Resistance

Polyurethane is more heat-resistant than spar urethane. Before you get confused, you should know that heat resistance is different from UV resistance. Heat causes paint to crack or split due to increased temperature. While UV rays cause paints to fade and discolor due to the presence of UV-B radiation.

In terms of heat resistance, polyurethane is better. That’s because it has very impressive flexibility that makes it contract and expand based on temperature changes. Spar urethane is also heat resistant but not as well as polyurethane. Spar polyurethane is even known to be very flammable and will catch fire when exposed to sufficient heat.

Indoor and Outdoor Use

Is the paint suited to exterior or interior environment?

Polyurethane is more suited to interior surfaces since the paint is not weather-resistant. Polyurethane will protect your tabletop from dents, scratches, and spills. But it will not protect your outdoor furniture from UV rays and storms.

Spar urethane however is more suited to outdoor use. Spar urethane is formulated with additives and resins that make the paint thrive in a harsh environment. Spar polyurethane will protect your furniture or wall from radiation, rainfall, and dents.

However, spar urethane can also be used indoors. To use spar urethane indoors, it’s best to go for water-based spar urethane. Also, polyurethane can be used outdoors but it’s best to go for exterior polyurethane.

Paint Cost

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

Ideal Paintbrush

What type of paintbrush is used to apply the paint?

Polyurethane and spar urethane can both be applied with a paintbrush. But, the difference in paint consistencies means you’ll need different types of paintbrushes. For polyurethane, you’ll need a bristled or foam paintbrush to apply the paint.

For spar urethane, you can only use a bristled paintbrush. This is because spar urethane is thicker and as such will reveal paintbrush marks if applied with a foam brush or non-bristled paintbrush.

Paint Thinning

Do you have to thin or dilute the paint to apply it?

With spar urethane, you have to thin or dilute the paint with a solvent before applying it. This is because spar urethane is very thick and applying the paint in its thick consistency will make paint application very difficult.

With polyurethane, you don’t need to thin it before applying. Though thinning is allowed, you can still apply the polyurethane without diluting the paint first.

Water-Resistance

How well does the paint thrive in humid or wet conditions?

Spar polyurethane is more water-resistant than polyurethane. Spar polyurethane as explained earlier was originally designed for the decks, hulls, and spar of sea vessels. Spar polyurethane can even be classified as waterproof. So for surfaces that will be exposed to moisture, spar polyurethane is the best.

Polyurethane on the other hand is also water-resistant but not as water-resistant as spar polyurethane. If exposed to moisture or wet conditions constantly, polyurethane will become blurred because moisture will sift into the paint coating through the sides.

Paint Drying

How quickly does the paint dry?

Spar polyurethane dries faster than polyurethane. This may come as a shock to you considering that spar urethane has a thicker flow. However, the reason spar urethane dries quicker regardless of the thickness of the paint is because spar urethane dries based on oxidization.

As soon as the paint is applied and exposed to air, it will begin to dry. Spar polyurethane is said to dry to touch in less than an hour.

Polyurethane however needs a longer drying time. For starters, polyurethane is produced from the chemical reaction between isocyanates or diisocyanates and alcohol or polyols. Also, polyurethane doesn’t dry based on oxidization. Polyurethane dries due to the evaporation of the paint solvent.

Since oils take longer to evaporate and Polyurethane is usually oil-based, you can expect Polyurethane to take longer to dry. However, you can speed up the drying time of polyurethane by using a heater, hairdryer, or heat lamp.

Paint Fumes and Toxicity

How strong or toxic are the paint fumes?

Both polyurethane and spar urethane produce paint fumes. But polyurethane produces more toxic fumes than spar urethane. Also, polyurethane coating tends to release an offensive odor even after the paint has dried.

Water-based polyurethane and water-based spar urethane are safer and tend to produce fewer fumes compared to the oil-based versions.

Now that you know the differences between polyurethane and spar urethane, which of the two do you need? Let’s take a closer look.

Related Read: Polyurethane vs Varnish?

Should I Buy Polyurethane or Spar Urethane?

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

What’s Your Budget?

Spar urethane is costlier than polyurethane. If you want a pocket-friendly option, it’s best to go for oil-based polyurethane. If you don’t mind the cost, you should go for water-based spar urethane. Though costlier, water-based spar urethane produces fewer fumes and is very durable.

Are You Painting Outdoors or Indoors?

If you are painting indoors, you should go for polyurethane as it’s suited to indoor surfaces. For outdoor surfaces, go for spar urethane. Spar is tougher and will thrive in the harsh exterior environment.

When it comes to picking between polyurethane and spar urethane, the cost and surface to be painted are usually the determining factors since both paints are very similar.

However, other things to consider include:

Toxicity – Spar urethane is less toxic.

Durability – Spar urethane is more durable and will be fitting for surfaces that will see heavy action. However, don’t use spar urethane on walkways or floors because the finish is not hard enough to coat floors.

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

Final Words

Overall, spar urethane and polyurethane are tough finishes and picking one can be a difficult decision. But I hope this post has made that decision easier.

Remember, polyurethane is more suited to interior surfaces while spar urethane is more suited to exterior surfaces.

Also, spar urethane dries faster but is more difficult to apply while polyurethane dries slower but is easier to apply. Ultimately, your choice depends on your taste and needs.

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