Spar Urethane vs Polyurethane: Which to Use?

Wood requires proper protection to prolong its life and keep it looking beautiful–and for that, the right wood finish is essential. Spar urethane and polyurethane are two of the most widely used protective products. But which one is best? 

The answer? It depends. There are pros and cons to each, and you should understand them before starting your project. To help you decide, I’m going to take an in-depth look at spar urethane vs. polyurethane. Keep reading to learn more. 

Feature	Spar Urethane	Polyurethane
Primary Use	Outdoor woods exposed to elements	Common finish for woodworking
Origin	Invented for use on boats	Synthetic resin made from polymers
Appearance	Glossy, semi-glossy, or matte. Keeps original wood color	Thick, can be semi-glossy or glossy
Protection	UV light, cracking, water, and elements	Elements, abrasion, and provides longevity
Fumes	Less fumes, better for environment	Strong fumes and intense odor
Price	More expensive	Less expensive

Spar Urethane

Spar urethane is a finish that is ideal for woods that live outside or are regularly exposed to the elements. It does an excellent job at resisting water, and it’s the finish most commonly used on wooden boat hulls. Spar urethane was actually invented for use on boats, though its use has become widespread in non-marine environments, too. 

It features a higher resin content than polyurethane, which gives it these impressive properties. It also has additives that resist damage from the sun, making it ideal for any outdoor furniture that receives lots of direct sunlight. 

Oil Based vs Water Based Spar Urethane

These are the two kinds of spar urethane, oil based and water based. 

Oil based spar urethane produces more fumes, but the tradeoff is that it affords better protection. At the same time, it’s less environmentally friendly. Water based spar urethane gives off fewer fumes, dries faster, and is better for the environment. 

What Does Spar Urethane Look Like?

Spar urethane is available in many finishes. You can achieve a glossy, semi-glossy, or matte stain with this product. Once applied, it keeps the original wood color intact and prevents the grain from fading. It will keep your wood looking like new for a long time. 

What Protection Does Spar Urethane Provide Wood?

As mentioned, using spar urethane prolongs the wood’s lifespan, and it also allows you to go for more extended periods without reapplying protection. Additionally, it’s unbeatable in terms of protecting against damaging ultraviolet light, and it resists cracking as the wood expands and contracts naturally. 

How to Apply Spar Urethane

You can use a brush or a roller to apply spar urethane. In some cases, a spray gun may be the right application option. 

It’s vital to note that this product often has an extremely viscous quality when you first buy it, making application challenging. To make the paint easier to work with, many people choose to use paint thinner. A paint thinner can also help prevent brush strokes from showing. 

Another thing to keep in mind is that spar urethane can harden rapidly, especially in warm, sunny conditions. 

Wiping Method of Applying Spar Urethane without Bubbles

Aside from brushes and rollers, the wiping method is another way to apply this urethane. It requires an old rag and a mix of fifty percent urethane and fifty percent mineral spirits. This combination makes the product thinner and more manageable.

Once you’ve prepared the material, dip your rag into the diluted solution, and wipe it on the surface. You’ll get a bubble-free application, though this method does require more coats. 

Pros

• Resistance to elements
• Ideal for outdoor surfaces
• No strong fumes
• Blocks damaging UV rays and light 

Cons

• Difficult to mix with oil based paints 
• Spotty quality 
• More expensive 

Perhaps the most significant con is that its issue mixing with oil based paints can lead to a strange color, especially once the paint dries. 

Spar Urethane vs Spar Varnish

Often considered the same, spar varnish and spar urethane are very different products. Spar urethane is much hardier and will last a lot longer than spar varnish. At the end of the day, spar urethane IS spar varnish with an added urethane plastic that acts as its main resin.

Polyurethane

Polyurethane is an extremely common substance that can be used to coat a wide variety of materials. It is a synthetic resin that is made from polymers that are transparent and resist abrasion, and it’s often found in paint. Though spar urethane has become more mainstream, polyurethane continues to be the most popular finish for woodworking. 

Its main advantages are that it moves with the object you apply it to, and it also resists the elements well. It offers excellent longevity and fantastic versatility. 

Different Kinds of Polyurethane

Polyurethane is available in water based and oil based versions.

Oil based polyurethane is thicker, and it requires fewer coats than water based versions to get the same results. 

On the other hand, water based polyurethane dries faster, smells less, and is better for indoor use. It’s also available in several different sheens. 

Oil based polyurethane is more durable and will more closely mimic some of the characteristics of spar urethane.

How Polyurethane Is Applied

Polyurethane requires lots of care during application to achieve a uniform look. Though you need fewer coats because it’s quite thick, it’s very easy to leave unsightly brush strokes that might need fixing later.

It should be well-mixed before application, though it’s critical to avoid shaking it, as this creates air bubbles that will be visible on the wood. 

You can apply it using a brush, with a spray (no dilution needed), or using the wiping method. You can even use polyurethane from spray cans! It creates a hard smooth finish once dry and leaves either a semi-glossy or glossy finish. 

Generally speaking, oil based poly will take longer to dry than its water based polyurethane counterpart. But, oil based polyurethane will provide a thicker coat.

Pros

• Delivers a smooth finish, even on metal and rubber surfaces
• Provides protection in harsh environments
• Resists oxidation
• Withstands heat and cold 

Cons

• Has toxic substances 
• Harder to remove 
• Takes a long time to dry 

The most significant drawback of polyurethane is the toxicity problem. The toxins this finish contains slowly leak into the environment, a problem in interior spaces where they can build up over time. 

Spar Urethane vs Polyurethane

Now it’s time to take a look at how these spar urethane and polyurethane compare across a range of features. 

Oil

Oil content is probably the most significant difference between spar urethane and polyurethane. 

The amount of oil in spar urethane is much higher, which makes it softer and more pliable. The high oil content is why this product is sometimes referred to as long oil varnish. The amount of oil in polyurethane is much less, which makes it harder and stiffer. 

Dry Time

Dry time is a vital consideration, and if you want to tackle your project faster, choose spar urethane. It outperforms oil based polyurethane in this category, which dries slowly and slows down your work process. Spar urethane’s fast drying time also makes it easier to apply multiple coats in less time. 

However, keep in mind that cold conditions or high humidity levels affect drying times regardless of the product. 

Application

I’ve mentioned that spar urethane is thicker than polyurethane, which means you can’t use a foam brush to apply it. Because it is so finicky, spar urethane is often better suited to individuals who have experience applying it. 

Polyurethane requires less careful application, though in both cases, you should be aware of what kind of strokes you’re using, the direction, etc. If you don’t do it correctly, you may be able to see the brush strokes, or you may cause bubbles to form.

Fumes

There is quite a difference in the fumes that these products produce.

Polyurethane gives off strong fumes and an intense odor. It’s essential to keep your workspace well ventilated when using polyurethane–otherwise, you can become dizzy or get a headache.

Fumes are less of an issue with spar urethane, though you should still make sure to ventilate your work area to avoid problems. 

Price

Both polyurethane and spar urethane are reasonably inexpensive, but spar urethane is a bit pricier than polyurethane.  

Which Is Best for Your Project: Spar Urethane or Polyurethane?

The answer depends mostly on the conditions the wood surface will be living in and the conditions of the space you’re working in.  

If your project will be exposed to direct sunlight, or if it will be in contact with moisture, spar urethane is the best choice. However, if you want to apply an oil based paint to the surface, you may want to move away from spar urethane. 

Polyurethane is a better choice for most interior projects. However, the pungent fumes are a factor to consider (see my safety tips below for more information about protecting yourself while painting). If you cannot properly ventilate your workspace, you should choose spar urethane. 

In the end, if you’re still unsure whether to use spar urethane or polyurethane, heading to your local hardware store and getting advice from professionals is an excellent idea. You can explain your project in detail and get tips that will allow you to achieve the results you want. 

Why Is Choosing the Right Product Important for Your Project?

Curious to know why it’s so critical to choose the right finish? 

I used wood in different ways and under different conditions. Much of the wood in my homes lives inside, such as coffee tables, dining room tables, and beds. Some items live outside, such as patio furniture, and wood is often used in marine settings where it’s exposed to lots of moisture. 

To ensure that my wood products last as long as possible (and look beautiful!), it’s critical to choose the finish that works best for the different conditions and ways I used wood. A wood patio set requires UV and moisture protection that your wardrobe doesn’t, for example. 

Spar Urethane and Polyurethane Safety Tips 

Though using wood finishing products is much safer than, say, using power tools, there are some essential safety tips to keep in mind before using spar urethane and polyurethane. These products are chemical products, which require a few special considerations before using them.

Here’s what you should know. 

Fire Safety

Due to the highly flammable nature of urethanes, your workspace should be equipped with a fire extinguisher. You should also make sure to keep any heat sources well away from the product. 

Proper Equipment and Clothing

Loose, comfortable clothing will help you avoid accidents in your work area. A mask and goggles are also a must to protect your lungs and eyes from dangerous fumes. 

Cleanliness

Ventilation is critical when working with urethanes, and keeping an interior space well ventilated will promote air flow to avoid a dangerous buildup of fumes. You can do that by opening windows and using fans.

Additionally, you should keep your work area free of obstacles, such as tools, which can cause accidents if left on the floor. And once you finish, ensure that all your products are stored properly. 

Conclusion

As you have seen, there is no clear winner in the spar urethane vs. polyurethane faceoff. Both are excellent products that deliver a beautiful finish and offer fantastic protection. Choosing the right one is a matter of carefully assessing your needs and working from there.