Spar Urethane vs Polyurethane: What’s the Difference?

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Overview

Spar urethane and polyurethane are urethane-based products used to seal, and or protect surfaces such as wood.  

Either product can be found in glossy, semi-gloss, and satin versions – with the gloss barrier being most common.

To help you better understand the nuances of each, our team has put together a guide breaking down spar urethane vs polyurethane. Additionally, we then highlight pros and cons, as well as provide a framework for when you should use each finish.

While they are rather similar products, they are often used quite differently. 

Spar Urethane

Spar urethane is a type of finish/ sealant that is primarily for outdoor use on wood that is exposed to the elements (sunlight, heat, water). It creates a protective barrier on the surface that prevents wear-and-tear from rain or moisture. 

Variants of spar urethane include spar varnish. A proper coat of spar urethane can last for approximately 10 – 15 years. Not surprisingly, this makes it a reliable surface coat for surfaces that get used often.

It consists of a hard resin and solvent-based solution that provides a thin plastic-like barrier.

Polyurethane

Polyurethane is a type of hard finish that can come in various formats including oil and water-based versions as well as a range of gloss levels. It is typically used to protect and provide a smooth surface to whatever material it’s applied to. 

Oil-based polyurethane tends to be thicker and requires less liquid per coat to achieve a proper seal. In contrast, water-based polyurethane tends to have less of a smell which can be helpful for indoor use.

Polyurethane may dry slower than spar urethane and can cause a smell. Make sure to use a respirator while painting with this finish. 


Spar Urethane Vs Polyurethane: The Differences

Spar urethane and polyurethane are rather similar products. They are both urethane-based finishes consisting of resin and solvent, used to seal a range of indoor and outdoor surfaces. 

The seals’ intended purpose is either to protect the surface from the elements, or to provide it with a smooth and unblemished finish. 

Both products can be found in a range of gloss levels, allow you to get the finish you desire.

Furthermore, most individuals apply both products with paint brushes, akin to how you may apply varnish or polycrylic.

With that being said, there are some fundamental differences between the products. These differences ultimately come down to the time to dry, coats required, and the flexibility of the finish.

Time to Dry

Spar urethane tends to dry faster than polyurethane, this is important if you require a fast-drying finish.

Coats Required

Spar urethane can also be thicker than polyurethane, which requires you to apply fewer coats to achieve your desired finish.

Flexibility

Polyurethane dries harder than spar urethane, which is known to have a more flexible finish.

This is believed to be due to the amount of oil in each product. However, this will also depend on the brand of polyurethane or spar urethane you buy.


Spar Urethane vs Polyurethane: Pros and Cons

To help you better understand the pros and cons of each urethane, we break it down by each finish below. 

This section should help equip you with the knowledge to determine the benefits of each for your project.

Pros

Spar Urethane

Spar urethane is commonly used in outdoor applications to protect surfaces from the elements including rain, abrasion, heat, and light.

It forms a protective barrier that prevents ultraviolet (UV) rays from damaging your surface, something that is especially important when using wood products.

It goes on thicker than polyurethane and typically requires fewer coats of finish. Additionally, it is less toxic, and is known to have fewer fumes than its counterpart.

With that being said, polyurethane has its benefits as well.

Polyurethane

Polyurethane is great for indoor uses like cabinets, desks, or frames. It provides a hard, abrasion-resistant surface that can hold up to moisture as well as wear and tear over time. 

In addition, it provides an extra smooth finish that looks great in a number of settings. Another positive, it can easily be applied by anyone using a paint brush.

People typically opt to go with polyurethane when they are seeking a seamless, ultra-smooth surface, as it does not hold up to moisture or heat as well its counterpart.

Cons

Spar Urethane

Spar urethane is more limited in its use cases, often being relegated to outdoor projects only. Furthermore, spar urethane is also more expensive than polyurethane. 

Both these attributes can lead people to choose polyurethane or even epoxy over this finish.

Polyurethane

Polyurethane tends to be more toxic than spar urethane. This has led some to avoid polyurethane out of concern for their health. When handling polyurethane, it is wise to use a respirator to combat the fumes that may arise.

In addition to these challenges, polyurethane usually takes longer to dry and is commonly less resistant to the elements, specifically heat and moisture, than spar urethane.


Spar Urethane vs Polyurethane: Which is Best?


Ultimately, there is no best, the finish you end up with will come down to the needs of your project. 

When shopping for either finish, whether online or at your local hardware store, make sure to read the label to ensure you know what you are getting and the guidelines for the product. 

With that being said, here is an easy way to determine which may be best for you. 

If you are looking to protect or finish something outdoors, then spar urethane is the better option. Its protective coat will help prevent rain or sun from corroding your surface.

Polyurethane is typically used indoors on surfaces like hardwood floors where you want a smoother, more refined appearance. Additionally, it provides an abrasion-resistant surface.


Conclusion

We hope that this guide helped you better understand the differences between spar urethane vs polyurethane. You should now be well equipped to determine what’s best for your next home improvement project.

While we talked a lot about the pros and cons of each urethane product, we didn’t touch upon their downsides relative to epoxy. Note, if you are planning on using either product in a high moisture environment (i.e. boat hull, water goggles, etc), you may be better suited by an epoxy.

If you have any questions about which is best for you, don’t hesitate to drop a comment in the box below and a member of our team will do their best to get back to you!

Ryan

Ryan

Ryan is a writer for ToolboxAdvice.com. He's been a hobbyist and painter for a long time, and now enjoys teaching others. His content primarly focuses on reviews and guides for painters and construction professionals.

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