Spar Urethane vs. Polyurethane: What’s Better for My Project?

Spar urethane and polyurethane are the two wood finish products that I have been using for a very long time.

Although the names are similar and both are derived from oil, they have some unique qualities that make them different enough. So, it’s important to know these differences before using any of these finishing products for your next project.

The key difference between spar urethane and polyurethane is that the former offers superior water resistance and performs well in high-humidity conditions whereas the latter provides stronger chemical resistance, including against ethanol, making it more durable in environments exposed to cleaning agents.

Think of spar urethane as the all-weather adventurer—built to withstand moisture and humidity where polyurethane may struggle to cure. However, it is more vulnerable to chemicals and alcohol, which can degrade its finish.

Polyurethane, on the other hand, isn’t as fond of moisture but stands tall against chemical attacks, including ethanol. So, pick your fighter based on whether you want a water warrior or a chemical champion.

Want to dive further in detail? Let’s break down further on each of these products along with the differences in terms of composition, durability, application, and best use cases.

What is Spar Urethane?

Spar varnish didn’t just get its name out of thin air—it comes from the sturdy wooden spars found on marine ships. These poles took quite the beating from wind, rain, moisture, and relentless sun, making them the ultimate test subjects for a tough-as-nails protective coating.

Designed to endure the unforgiving high seas, this powerhouse blend of resins and solvents creates a rock-solid finish that shrugs off wear and tear like a seasoned deckhand. Not only does spar urethane add a sleek, polished look, but it also boasts an almost superhero-like resistance to UV rays.

In simplest terms, spar urethane is a finish applied to wood products exposed to elements like moisture and sunlight. Because it resists water so well, you’ll also often find it used on submerged wood surfaces such as the hulls of boats and the like. If durability and weather resistance are a priority for your woodwork, this varnish delivers unmatched performance.

Types

There are two primary types of spar urethane: oil-based and water-based.

Both types provide a durable, protective finish that shields wooden surfaces from scratches, dents, and chemical spills. However, oil-based spar urethane differs in several key ways—it dries more slowly, emits a stronger odor, and develops a yellowish tint over time.

Despite these differences, both formulations offer exceptional protection and longevity for wood exposed to the elements.

Pros

• Easy to apply
• Quick drying time
• Scratch and water-resistant
• Contains UV blockers for added protection

Cons

• Does not dry as hard as other finishes
• Requires occasional reapplication to maintain protection

What is a Polyurethane Finish?

Polyurethane is a type of plastic finish often used for wood surfaces. It’s a tough, water-resistant finish that creates a hard, durable layer, offering outstanding protection against water, scratches, and daily wear.

It’s a go-to choice for surfaces like floors, tables, decks, and fences, where durability is key. You can also use it to protect and enhance the appearance of tables, chairs, and cabinets (both inside and outside). However, because of its thicker texture, it’s typically avoided for high-end furniture that calls for a more polished and refined finish.

Types

Polyurethane also comes in two main versions: oil-based and water-based.

Both do the job, but each has its quirks. Oil-based polyurethane gives the wood a rich, warm glow but tends to yellow over time—like a well-aged whiskey. Water-based polyurethane, on the other hand, dries clear and stays clear, perfect for those who prefer their wood to keep its natural beauty.

With proper application and maintenance, either version can protect your wood for over a decade.

Pros

• Versatile in application—brush, spray, or roll your way to perfection
• Thick and tough to last longer
• Holds off oxidation
• Dries hard enough to handle the rough and tumble of daily life
• Scratch, chemical, and water-resistant—basically, a bouncer for your wood

Cons

• Takes Longer to Dry
• Difficult to Remove
• Emits high levels of VOCs

Spar Urethane vs Polyurethane: Differences

It should be noted that both these topcoat products contain many similar elements, lightweight, and flexible. Also, they provide a strong, resilient finish that resists impacts, chipping and the like.

But even with all the similarities, there are some noteworthy differences which we will discuss here in detail…

1- How easy is it to apply:

The application process for both products is essentially the same: scuff up the surface, give it a good clean, slap on the first coat using a brush, wait for it to dry, add another coat, and repeat if necessary.

However, some folks claim spar urethane when applied correctly has the edge because it hides brush strokes better than polyurethane. And they’re not wrong – spar urethane’s softer nature allows it to level out more easily.

But here’s the catch: this is only true when you’re working with oil-based polyurethane. Water-based polyurethane doesn’t show brush strokes or dust nibs nearly as much as its oil-based sibling.

So, while spar urethane may have a slight edge in some areas, more people are now leaning towards water-based poly, which evens out the playing field. At the end of the day, the ease of application remains pretty much the same.

2- How toxic are they:

Arguably the most noticeable difference lies in the oil concentration and toxicity between spar urethane and polyurethane.

Polyurethane is loaded with isocyanates and VOCs (volatile organic compounds), some of which are known carcinogens like Benzene which can irritate your eyes, lungs, and skin. Yikes! So, if you’re using oil-based polyurethane, extra caution is a must – but once it dries, it’s no longer a threat.

Now, spar urethane isn’t completely innocent either. It still emits VOCs, though not as many as polyurethane.

So, in the battle of toxicity, spar urethane comes out as the lesser evil. Both water-based and oil-based spar urethanes contribute to VOC levels, but typically, spar urethane is safer for both people and the planet.

3- How fast do they dry (drying times):

Polyurethane is infamous for taking forever to dry, sometimes a full 24 hours between coats. Spar urethane? Just 4 hours. Sounds like a no-brainer, right?

Well hold on – not so fast. That comparison only applies to traditional oil-based polyurethane.

The fast-drying version, which dries in a speedy 4-6 hours is quite the leap, and with many brands now only selling fast-drying oil-based polys, the old rules don’t apply.

As for the water-based versions? Both are ready for a new coat in 2 hours, with identical curing times. And don’t forget about one-coat and wipe-on polys, which dry even faster than spar urethane.

4- Which is more durable of the two:

Here’s where things get tough.

Polyurethane is one of the toughest finishes for interior woodwork, thanks to its hard-drying resins and solvents. Whether it’s oil-based or water-based they both are built to last. Some say oil-based is tougher, but that’s just old-school thinking.

Spar urethane, while still tough, isn’t as hard as Polyurethane. It’s made to flex with temperature changes outdoors, so it’s a bit more forgiving. But this means it’s not as durable – Polyurethane can last 1-2 decades, while spar urethane is lucky to hit 5 years for water-based, or up to 15 for oil-based.

When it comes to longevity, there’s also the dreaded ambering issue. Oil-based Polyurethane turns yellow with age, which looks fine on dark woods but can be a dealbreaker on lighter ones. Water-based Polyurethane stays crystal clear, but water-based spar urethane can still be yellow, especially on lighter stains. So, if you care about clarity, make sure to test it out first.

5- Which one provides better protection from elements:

Originally made for boats, Helmsman spar urethane is a champ at handling the elements. It can take on UV rays without graying your wood and stands up to water like a pro.

However, spar urethane is a bit softer than Polyurethane, so when exposed to lots of moisture, it can turn milky. Don’t panic – it clears up once it dries.

Polyurethane, on the other hand, doesn’t have that issue, but it can’t stand up to moisture and sunlight as well. Only exterior Polyurethane usually comes with UV blockers, and interior versions are no match for spar urethane’s outdoor toughness.

So, if you’re dealing with wood that’s getting daily sun exposure, spar urethane is your best bet.   

Here’s a comparison table highlighting the key differences between Spar Urethane vs. Polyurethane.

Feature	Spar Urethane	Polyurethane
Ease of application	Hides brush strokes better due to its softer nature.	Easier with water-based formulas; oil-based shows brush strokes more.
Toxicity	Emits fewer VOCs and is safer for both people and the planet.	Contains isocyanates and VOCs, which are more toxic.
Drying time	Oil-based: 4 hours, Water-based: 2 hours.	Oil-based: 24 hours, Water-based: 2 hours. Fast-drying versions take 4-6 hours.
Durability	Less durable, can last 5-15 years depending on type.	Highly durable, lasting 1-2 decades. Oil-based is tougher.
Protection from elements	Excellent for outdoor use, UV protection, water resistance.	Less effective against UV and moisture, unless exterior version.
Clarity	Some water-based versions may turn yellow over time, especially on light woods.	Water-based remains clear, oil-based may yellow on all surfaces.
Best for	Outdoor use, UV and moisture resistance. Not suitable for floors or hard surfaces like tabletops.	Suitable for indoor use, high-traffic areas, floors, furniture, and many other wooden surfaces.
Price	Higher price.	Relatively inexpensive.

Spar Urethane OR Polyurethane: Which is Better for Your Project?

Depending on the surface you want to cover, it may be difficult to know which one is right for your needs.

• Spar urethane is better for surfaces that are exposed to the elements particularly those that will be underwater.
• For interior surfaces, polyurethane is generally better. However, be mindful that it does create toxic fumes. So, you will need to apply the substance in a well-ventilated area while wearing the appropriate protection, particularly a mask that covers the nose and mouth.

If you cannot apply polyurethane to a surface while being in a well-ventilated area, an example would be a room or basement with poor ventilation, then you should consider spar urethane as a good substitute.

You will still need to wear a mask, but it does dry faster and creates fewer fumes compared to polyurethane.  

Feeling still confused? Well, getting advice from experts such as those at your local hardware store can be helpful. They understand the basic use of spar urethane and polyurethane and which surfaces are best suited for either product.

Plus, if you are covering a surface on an item that may be moved from indoors to outdoors from time to time, then they can provide answers for that as well.

Related FAQs

Can You Put Spar Urethane Over Polyurethane?

Spar urethane can be a bit tricky to work with, but it adheres well to different finishes including polyurethane provided the old finish is completely dry. Just be safe—test a small hidden spot before committing to the whole project.

Also, be aware that applying polyurethane over spar urethane isn’t recommended. Polyurethane (especially oil-based) has a lower oil ratio than spar urethane, making adhesion unreliable. The best approach is to sand down the old finish and reapply the same type for a smooth, hassle-free result.

Guys at Benjamin Moore and Varathane also suggest that always sand the old finish and put the same as what you already have. This will deliver the best finish without extra effort.

Polyurethane vs. Lacquer: What’s the Difference?

Polyurethane is a plastic-based resin that forms a rock-solid protective shield, laughing in the face of scratches and stains. Think of polyurethane as the bodyguard of finishes—tough, durable, and ready to take a hit. Just be warned: oil-based versions can develop a yellowish tint over time, like an aging action hero.

Lacquer, on the other hand, is the sleek, stylish sprinter of the bunch. Made from nitrocellulose or acrylic compounds, it dries at lightning speed and stays crystal clear. But while it looks effortlessly smooth, it’s not as battle-ready as polyurethane—it bruises easier.

So, if you want an armor-clad fortress, go for polyurethane. But if you’re after a quick-drying, glassy-smooth finish, lacquer is your flashy best friend.

Is There a Difference Between Urethane and Varathane? 

Urethane is a type of polyurethane, while Varathane is a brand name for a specific type of urethane product. Both are finishes that are applied to wood surfaces to protect them from scratches, nicks, and other types of damage.

Then there is also a Polycrylic which is a water-based product that contains acrylic resins. It is less durable than polyurethane and will yellow over time.

The Bottom Line

Choosing between spar urethane and polyurethane comes down to where and how your project will be used. Spar urethane shines outdoors, offering top-tier protection against moisture and UV rays – think of it as a weather warrior. Polyurethane, though, is your go-to for indoor spaces, giving furniture and hardwood floors that durable, glossy finish that says, “I’ve got this.”

Don’t forget to factor in your aesthetic goals and budget. Spar urethane costs more and needs a bit more TLC due to the elements. But if you’re gearing up for outdoor exposure, it’s worth every penny. Polyurethane is cheaper and easier to care for, making it perfect for indoor use, where you want that shiny, long-lasting appeal without the hassle.

So, pick wisely based on the environment, the look you’re going for, and your willingness to maintain it. It’s all about finding the right finish for your needs and wallet.