Spar Urethane vs. Polyurethane: What’s the Difference?

If you’re looking to buy a product to protect your furniture, you’ll likely come across two main options: polyurethane and spar urethane.

Spar urethane and polyurethane are both used as protective coatings on a variety of surfaces, primarily, wood.

It may seem like they’re essentially the same thing, but they have some differences that are important to understand.

Spar urethane is a super flexible, highly water and UV-resistant seal that’s easy to apply and typical for outdoor protection.

Polyurethane is a thermoset polymer with good tensile strength, exceptional chemical resistance, and a cost-friendly seal common for indoor protection.

These two substances have different chemical compositions. They are also manufactured differently, so their physical properties vary as well.

Let’s have a closer look at each of these seal materials so it’ll be easy to compare them.

Spar Urethane

Spar urethane is a protective finish sprayed on surfaces to protect against rain, moisture, temperature fluctuations, chemical degradation, and UV damage.

Spar urethane maintains the protective, hard, shiny surface on the surface it’s applied on for an extended period.

It is a premium-grade, resin-based material applied to wood to improve its look and protect it against damage.

Unique chemical properties

Spar Urethane is characterized by:

Types of spar urethane

There are two main types of spar urethane: oil-based and water-based. Depending on the purpose of your project, you may want to choose one over the other.

Oil-based spar urethanes have a longer shelf life and protect against heat and moisture.

However, they contain volatile organic compounds (VOCs), which can cause an unpleasant odor when applied and emit small amounts of harmful fumes into the air.

Water-based spar urethanes produce a thin, high gloss, durable finish. They’re ideal for clear coating all types of surfaces and won’t yellow or pit like oil-based spar urethanes.

They’re more environmentally friendly and much more user-friendly than their oil-based counterparts.

On the side, spar urethanes can be classified depending on the amount of shine they offer when applied to a surface:

How does spar urethane work?

Spar urethane uses a multi-coat process that uses the existing wood grain on the surface to create a durable finish with a realistic wood grain look.

For water-based spar urethane, the water evaporates, and the solid material remains as a thin protective coating on the surface it’s applied to.

Uses of spar urethane

Spar urethane is an incredibly versatile type of material that can be used to protect a wide range of surfaces, including:

Thanks to its high UV, sunlight, and moisture resistance; spar urethane is also ideal for outdoor furniture.

Although spar urethane is durable enough and great for the above areas, it’s not ideal for some uses. For example, it’s not advisable to use spar urethane on wood floors as it can’t withstand too much foot traffic.

Application process

When applying a spar urethane, the first and most important thing is to make sure the surface is clean and dry.

If it isn’t, you’re going to have a bad time. You also need to make sure that you don’t apply it in direct sunlight or on hot surfaces because it’ll not cure properly.

Dilute your highly viscous spar urethane with an equal part of mineral spirits or naphtha, or paint thinner.

The good thing about spar urethane is that it cures quickly into a hard, durable protective coat. The final hardness develops over time. You should wait for at least three days before subjecting the surface to everyday use.

Durability

Spar urethane offers a unique combination of both properties, which results in a tough, resilient surface coat that can handle the wear and tear of everyday usage while still retaining its original shape.

This makes spar urethane ideal for applications where relatively long surface protection is required.

Please note: Spar urethane doesn’t hold up well to excess or continued pressure, so its durability is not very good on high foot traffic rough or abrasive surfaces.

Safety concerns

Spar urethane, especially the oil-based variety, releases volatile organic compounds in the form of fumes. Oil-based spar urethane also produces a pungent smell that you don’t want to continuously be exposed to.

Due to these safety concerns, you should read and follow all the safety precautions indicated on your specific product’s packaging.

Polyurethane

Polyurethane is a protective finish applied to the surface of the wood to provide surface protection against various elements and for decorative purposes.

The finish is primarily designed to protect the wood from water, fungi, mold, mildew, and physical stresses.

Unique chemical properties

Just like other elastomeric compounds, polyurethane has its own unique chemical properties.

The following are some of the most important chemical properties of polyurethane:

Types of polyurethane

When it comes to polyurethane seal, you have oil-based vs. water-based options. Both have their pros and cons, but that depends on the project.

Oil-based polyurethanes dry slower than water-based ones, and they have a pungent smell. Oil-based finishes are great for heavy-use items because they have a longer lifespan than their water-based counterparts.

On the other hand, water-based finishes dry faster than oil-based ones and are great for extensive surface protection.

How does polyurethane work?

Polyurethane is a two-part material. Its main ingredient is polyols. In other words, polyurethane is made up of molecules called polyols.

When the two parts of the polyol mix, they form small molecules known as diisocyanates.

These diisocyanates react with themselves and link into chains, forming long molecules known as polyurethanes.

The long-chain polyurethanes bond with each other and bind with oxygen to form the hard surface we see when we apply the product to a surface.

Uses of Polyurethane

Polyurethane is typically applied as a surface coating to any smooth, hard surface that needs protection. This includes but is not limited to:

Although polyurethane can be used in both indoor and outdoor applications, it’s most suitable for indoor uses where extensive surface protection is needed.

Application process

The first thing you need to do is wear a mask or respirator that can bar volatile organic compounds released by poly and ensure there’s enough ventilation as polyurethane is smelly.

With these measures, you can proceed with the following steps:

The drying time for polyurethane seal is typically 3-4 hours.

Precaution: When applying oil-based poly, ensure the area is free of any heat source as the product can easily catch fire.

Durability

Polyurethane is a highly durable seal material and can last for years without showing much wear. In fact, poly is the best protective coating for high-traffic areas such as wooden floorings.

Please note: The key to a long-lasting polyurethane seal is to use a good product in the first place.

For this reason, you should only get your poly from a trusted and reputable store.

Safety concerns

Polyurethane is a common material for furniture as its incredibly durable, water-resistant, and cheap.

Unfortunately, manufacturers are averse to sharing the risks of this seal with consumers.

The risks associated with polyurethane usage become more apparent when we analyze the components of this material.

Many kinds of polyurethane contain harmful volatile organic compounds VOCs that are a health risk and a pollutant to the environment.

You can only protect yourself from these risks by taking all the necessary precautionary measures, such as wearing a respirator, working in a well-ventilated space, and staying off areas with a heat source when working with poly.

Differences between Spar Urethane and Polyurethane

Polyurethane vs. Spar Urethane: Which one should you use?

For indoor surfaces, polyurethane is more suitable, but you need to apply it in a properly ventilated area and when wearing the proper protective gear.

You should also use polyurethane on high foot traffic surfaces requiring more sturdy protection.

Spar urethane is more suitable for surfaces exposed to outdoor elements such as moisture and sunlight.

What number of coats of spar urethane and polyurethane is enough?

Anywhere from two to four coats. You need enough to give the surface an excellent gloss, but not so much that the wood becomes too hard.

Generally, three coats is a good number. It all depends on the look you are after, the wood you’re working with, the application technique you’re using, and how much time you want to spend on finishing it.

Between spar urethane and polyurethane, which one scratches easily?

Spar urethane is more susceptible to scratching than polyurethane, though both offer good abrasion resistance.

Polyurethane is a thermoplastic polymer known for its toughness, flexibility, and stability. Polyurethane has a low coefficient of friction and good abrasion resistance than spar urethane.

Can you use polyurethane over spar urethane?

Spar urethane has a higher oil ratio, so polyurethane can’t be used over it.

Polyurethane takes much longer to completely dry once applied compared to spar urethane, and these differences in properties can lead to issues with surface texture and gloss.

It’s always recommended that you go with a single seal to get the best results.

Final thoughts

Both polyurethane and spar urethane will coat your furniture in a layer of protection that seals the wood fibers from moisture and other elements.

But each of these products has its own set of advantages and disadvantages. For example, spar urethane provides better UV protection than polyurethane but is susceptible to cracking.

The differences between the two seals make one more suitable than the other for your project.

By reviewing this article, you should have a better understanding of both materials and be able to choose which is most appropriate for your project.