Spar Urethane VS Polyurethane: Know Difference Between Them?

Woodworking is fun! You put your knowledge and experience into coming up with amazing designs that stand out. And, if you’re in business, you make sales and ensure each of your customers leaves happy.

As you most likely agree, durability is one of the most important factors when it comes to creating or maintaining furniture. In that regard, I reckon you already know how critical finish is.

Browsing finishes online or at your local hardware store can leave you confused as to what to purchase. Speaking of that, you might be wondering – what is the difference between spar urethane and polyurethane?

Both finishes are popular choices, but which one should you get?

If you’ve found yourself scratching your head because of this question, you’ve come to the right place. I will walk you through this comprehensive  polyurethane vs spar urethane guide and help you select the right finish.

Let’s get started.

Why Choose the Right One Between Spar Urethane and Polyurethane?

Wondering why the knowledge of the two finishes matters?

We use furniture items differently. Some are for indoor use, such as wardrobes, coffee tables, and beds. Others are left outside, such as the porch seating. Yet others are used in marine environments and other areas where they’re more exposed to elements.

Now, finishes work differently. While some provide moderate protection from elements, others go a mile further to deal with harsher elements like winds, sunlight, and the salt water of the oceans.

So, as you can see, selecting the right finish matters.

Let’s now dig in and find out what the deal is with these two compounds.

What Is Spar Urethane?        

Ever thought about how the spars of a boat/ship are able to hold up to all those elements in the waters? I am talking about the poles that hold the sails in place to steer the boat.

So, how are these spars able to hold up in the midst of all the moisture, wind, salt, and sunlight?

Well, that is what spar urethane was invented for. It creates a sturdy coating that efficiently keeps the spars protected and hence intact for longer.

For a while now, boat owners have enjoyed this benefit. But today, spar urethane has crossed niches to be used in non-marine circumstances.

The secret of this substance is that it contains a higher content of resins than solvents as compared to polyurethane. Moreover, it is infused with additives that disperse the harmful rays from the sun.

It’s no wonder carpenters who deal with outdoor furniture love it so much. Spar urethane is also rising as a finish of choice for items that will be exposed to lots of direct sunlight.

Another feature of the finish is that its consistency is thicker as compared to that of the typical polyurethane. And because of that, the product dries a lot faster. All you need is expose it to air, and you’ll see it dry faster than polyurethane.

That happens through a process called oxidation.

There are two kinds of spar urethane. There is an oil-based one and there is a water-based one. The oil-based spar urethane releases more fumes but it provides more reliable protection.

However, for someone who cares deeply for the environment, the water-based option would be the right pick. It releases less fumes and it also dries faster, by the way.

Pros:

• Great exterior varnish that works for windows, sheds, doors, etc.
• High resistance to UV rays, hence durable
• Can be wiped onto a surface or applied with spray gun after getting diluted
• Produces less harmful odors when being applied
• Doesn’t darken with time
• Friendlier to the environment than polyurethane

Cons:

• Viscous, thus difficult to apply without diluting it first
• Can dry too quickly if applied in warm conditions, leaving little room for fixing mistakes
• Relatively more expensive
• Flammable, thus shouldn’t be applied near heat sources
• Not very resistant to chemicals and alcohol

What is Polyurethane? 

For those of you that are used to woodworking projects, polyurethane is likely a quite familiar product. It is more popular than spar urethane.

But what exactly is it?

Polyurethane is made up of synthetic polymers. These are plastics that contain long-chain molecules. They are transparent and rather abrasion-resistant.

Although a coating of polyurethane finish is strong, it is also flexible. Like spar urethane, it’s able to move with the object that you apply it onto, though not as efficiently.

As a synthetic resin, polyurethane is able to resist elements like fungus, mildew, and water efficiently.

Concerning versatility, this finish is the most preferred in the market. It comes in a variety of forms. The main choices are – water-based or oil-based. Aside from that, you get to choose from a range of sheen levels.

If you’re looking for a tough final coat that offers longevity, you definitely want to think about oil-based polyurethane for your wood project.

The oil-based option offers you a few benefits. For instance, it’s thicker, therefore needing fewer coats. On the flip side, it needs more care during application. The chances of leaving brush strokes and failing to achieve a smooth finish are more when using oil-based polyurethane.

This oil-based product is made from a range of chemicals mixed together. Unfortunately, these chemicals tend to darken over time. Moreover, they release fumes that are a little hostile to the environment.

Looking for something that is friendlier to the environment? Go for the water-based polyurethane. It is also less smelly, meaning you can apply it to wood installations meant for indoor use.

Pros:

• Can be sprayed on directly without dilution
• Less expensive
• Dries slower, giving you the time to work it to a smooth finish
• Not flammable, hence you can use it near sources of heat
• Highly resistant to alcohol and other chemicals

Cons:

• Darkens with time
• Not very resistant to UV rays, hence not the best choice for outdoor wood
• Less friendly to the environment
• Smellier during application – ventilation is needed when applying the oil-based option indoors
• Bubbles might be present is the final coat if the product is shaken – it should be stirred instead
• Takes more time to dry

Spar Urethane Vs Polyurethane: Head to Head Differences Between Them

How many differences between polyurethane and spar urethane have you noticed so far? I believe you’ve made out a few.

Well, the variations between the two finishes range from the chemical composition to the application process used. Understanding what sets the finishes apart will come in helpful as you will know when one is a more suitable finish than the other.

Let’s take a closer look.

Oil content

The amount of oil that each of the products contains is one of the major deviations.

You know why spar urethane is softer and more pliable? It’s because it has more oils mixed in than the polyurethanes.

The higher oil content adds to the flexibility of the application. Hence, should the wood expand or contract, perhaps due to changes in temperature, there are lower chances of the application cracking or flaking.

Polyurethane has a lower oil content. Thus, it is less flexible than its counterpart. But that doesn’t mean it doesn’t hold up. It does, only to a lower extent. Polyurethane works as a nice glossy polish, but if you want more flexibility, it’s better to use spar urethane.

Drying time

Want to be able to complete your project quicker? Go for spar urethane. It dries much faster than polyurethane. Thanks oxidation, this product starts to dry as soon as it is exposed to air.

Apart from completing projects faster, applying multiple coats becomes more practical when using spar urethane.

If you’re only getting started with finishes, chances are that you’ll need more time to bring the application to a smooth consistency. That means you need something that dries slower – polyurethane. The downside is that slow drying means taking more time to complete the work.

If the air is cold or the humidity is high, the drying time goes up farther.

Moreover, there’s an increased risk of the work getting contaminated with dust and other particles.

Application process

The finishes have different consistencies in that spar urethane is thicker than polyurethane.

That means the application process and gadgets have to vary slightly.

While you can apply both with a brush, you cannot use a foam brush to apply spar urethane. You can only use a bristled brush. Why? The thicker consistency increases the risk of leaving strokes, which you get to avoid more efficiently with a bristled brush.

It makes sense that spar urethane is more suited to the experienced folks. Wondering why? You see, the product dries quicker and as I have already said, it has a thicker consistency, which means extra care needs to be taken during application not to leave brush marks.

With polyurethane, you needn’t be very careful as it is easier to achieve a smooth finish. Another upside is that you can use any application method. You can use either a foam brush or a bristled one. Besides, there are spray options that you apply directly without dilution.

If you want to apply spar urethane, you must first dilute it to thin it.

Fumes

Both products have fumes. Only the water-based polyurethane tends to not release fumes.

That being said, the fumes that polyurethane releases are more harmful than the ones that spar urethane releases. That’s why spar urethane is considered environmentally-friendlier than its counterpart.

On the other hand, spar urethane is flammable while polyurethane is not very flammable.

Price

It is undeniable that these two finishes share many similarities. For instance, both are flexible, only that one is more flexible than the other.

For that reason, price steps in as a major deciding factor for many people.

Spar urethane is generally more expensive than polyurethane. But, sprayable polyurethane tends to be pricier.

Safety Tips to Remember When Using Polyurethane and Spar Urethane

The truth is, both of these finishes are chemical products. Even though spar urethane has a higher content of natural substances, it contains additives, which are chemicals.

And we know one thing about chemicals – they’re not very friendly. That means we should be careful when working with them.

Here are a few safety tips to keep in mind:

Put on the Right Gear

These finishes produce fumes. Polyurethane’s fumes are more harmful, but whichever of them you’re using, be sure to wear comfortable, protective clothing. It’d be good to have a mask and goggles on.

Be Prepared to Deal with a Fire

This is especially important when working with spar urethane, which happens to be more flammable. I am not saying there will be a fire, but keep away sources of heat and have a fire extinguisher close in case there’s one.

Install ventilation

If you’re not working outside, it’d be advisable to have some form of ventilation. That way, you’ll be able to beat out any fumes that are produced.

Conclusion

If you’re looking for a fine finish quality, both polyurethane and spar urethane would be good considerations.

While these compounds serve the same function, there are differences as you’ve seen in this polyurethane vs spar urethane comparison post.

Mainly, spar urethane contains a higher oil content and takes less time to dry. Go for it if you’re more experienced with finishes and be sure to produce a nice finish in a short duration.

Again, if you’ll be working in an area that’s not very much ventilated, such as a basement, spar urethane is more appropriate.

Polyurethane is more suited to those who are learning to deal with finish as it gives you more room to correct errors. It is also cheaper and can be applied using any brush or sprayed on without dilution.

Lastly, in case you’re looking to apply the finish to a place that is often exposed to alcohol or harsh cleaning chemicals, such as a bar counter or bathroom furniture, you better use polyurethane. It is more resistant to alcohol and chemicals.