Best Exterior Polyurethane for Wood

By MWB-Team •  Updated: 09/01/23 • 

When wood is used outdoors, it is exposed to different elements, such as ultraviolet light from the sun’s rays, rain, and changes in temperature and humidity. Getting the best protection should protect your wood from surface graying, rotting, and other defects caused by exposure to outdoor elements. This article reviews our best polyurethane for exterior use and a buying guide to help you make an informed decision.

Our Best Exterior Polyurethane Picks

Our Best Exterior Polyurethane

1. Rust-Oleum Ultimate Spar Urethane

Rust-Oleum Ultimate Oil-Based Spar Urethane
$25.35

The Rust-oleum spar urethane is a good finish that protects outdoor wood projects and furniture. It holds up well under the sun, rain, and temperature changes. Overall, this is one of the best exterior polyurethane out there.

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Apart from outdoor paint and stain sealers, the Rust-Oleum spar urethane is one of the few exterior polyurethane that does a very decent job for outdoor wood projects. It has superior water and UV protection and goes smoothly and in thicker coats. Once you apply the first coat, I would leave it overnight before you apply the second coat.

Although it goes thick, one application cannot offer the best protection. We recommend going with two to four coats for the best results. After applying each coat, remember to sand softly for a better-looking finish. It comes in gloss and satin sheens for a customizable appearance.

Overall, the Rust-oleum spar urethane works and holds up much better than other types of exterior polyurethanes. Whether it’s UV rays, temperature changes, rain, and much more, this is a great product to protect your outdoor wood project.

Another Great Option

2. Minwax Helmsman Indoor/Outdoor Spar Urethane

Minwax Helmsman Spar Urethane
$25.78 ($0.81 / Fl Oz)

The Minwax spar urethane is a durable finish to protect your wood outdoors and indoors. It is available in an oil-base, best for the outdoors, and water-base, best for indoors. It comes in different sheens, from satin, semi-gloss, and gloss.

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Minwax is one of the biggest brands when it comes to wood finishes. The Minwax Helmsman is a spar urethane that works indoors and outdoors well. It is available in water-based, oil-based, and aerosol forms. It also has different sheens to match your preferred finish appearance better.

The Minwax spar urethane is easy to apply and looks great if you apply it right. It smells like most oil-based polyurethanes in this list, so it’s better to use it outside. However, water-based spar urethane is better for indoor use and suitable for outdoor use.

The spar urethane also dries in a good amount of time, allowing you to apply at least two coats daily. Overall, the Minwax spar urethane goes a long way and looks amazing after drying up.

Best Exterior Water-Based Poly

3. General Finishes Exterior Topcoat

General Finishes Exterior 450 Water Based Topcoat
$42.00

The GF Exterior 450 is a water-based polyurethane that dries into a clear top coat. It is easy to apply and performs better against ultraviolet light, temperature changes, and other elements. However, it is more expensive compared to other exterior polyurethanes.

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While we prefer oil-based polyurethane for the outdoors, we were pleased with the results of this GF exterior product. Compared to other exterior finishes on this list, this is much more expensive, but the results are well worth it.

It is easy to apply and goes on in thin coats, so you’ll need around three or four coats to look great. The water-based polyurethane dries quickly and self-levels to leave behind a smooth and flawless-looking finish. It also dries into a clear finish, which is good if you’re looking for a finish to preserve the natural appearance of wood.

Other than the price tag of this water-based polyurethane, the results are amazing. It also offers good performance with UV absorbers that minimize fading under the sun’s rays. The faster drying time also means you’re done finishing your project the same day.

Exterior Water-Based Spar Urethane

4. Rust-Oleum Ultimate Spar Urethane

Rust-Oleum Ultimate Water-Based Spar Urethane
$25.68

The Rust-oleum water-based spar urethane goes on easily, dries fast, and does not alter the color of the wood. It is also low in odor, and the overall results are good when used outside.

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While polyurethane offers the best performance and durability, spar urethane improves the qualities of polyurethane and is best for use outdoors. It goes without saying, a water-based spar urethane, like this one, will hold up much better outdoors compared to normal water-based polyurethane.

It has a milky color in the can and dries to a clear and hard finish. It goes on easily, but each coat is very thin. You’ll need at least four coats to build up better protection. The good thing is that it dries fast, so you can apply up to three coats or more in a day.

The results are fantastic, and it takes less time to apply. While it may not last ten years, it is cheap, easier to apply, faster to dry, and looks great. Overall, we highly recommend this if you’re looking for a faster-drying spar urethane that does not alter the color of your wood and has good performance outdoors.

Exterior Polyurethane Buying Guide

Oil-Based vs. Water-Based Polyurethane

Oil-based polyurethane uses mineral and petroleum solvents as the vehicle for the polyurethane resins. Water-based polyurethane use water as its solvent for carrying the polyurethane solids. Apart from the solvent used to mix oil and water-based polyurethane, their appearance and durability are also different.

Oil-based polyurethane imparts a warm amber hue to the wood, which continues to yellow more over time. Water-based poly dries to a crystal clear appearance and does not yellow over time.

In terms of durability, oil-based polyurethane has an edge. It is better suited for an outdoor environment compared to water-based polyurethane. Oil-based poly also makes for a good finish for more exposed surfaces or at a higher risk of getting damaged, such as floors, tabletop, and more.

Varathane Water-Based Spar Urethane

Varathane Water-Based Spar Urethane

Water-based polyurethane is also still durable and can be applied as a protective finish to various surfaces. However, use water-based poly for surfaces that will not be exposed to much usage, such as bookshelves, display tables, and other surfaces.

For an in-depth post on their differences, check out our oil-based vs. water-based polyurethane guide. For their difference in drying times, check out our polyurethane drying time article for more information.

What is Spar Urethane?

Spar urethane is a type of polyurethane designed to stand up to different elements such as high moisture, UV light, wind, and much more. It is derived from spar varnish, which got its name from the wooden poles (spars) on marine vessels, where it was used to protect them against unusually high moisture, wind, and sun exposure.

Varathane Oil-Based Spar Urethane

Varathane Oil-Based Spar Urethane

Spar urethane, like spar varnish, has a high resin content, which gives it a yellowish color. However, spar urethane includes urethane plastic as one of its building resins, thus its name.

Spar Urethane vs. Polyurethane

Both spar urethane and polyurethane are durable finishes you can use to protect your wood projects. However, for outdoor use, spar urethane is a better option. It has a high amount of oil, making it more flexible when used outside than polyurethane. As the wood expands, spar urethane also expands, making it less likely to chip or crack.

Outdoor Bench Finished with an Early American Stain Clear Semi-Gloss Spar Urethane

Outdoor Bench Finished with an Early American Stain Clear Semi-Gloss Spar Urethane

Spar urethane also has a high resins content and a higher amount of added additives that help disperse ultraviolet light. This makes it better suited to handle outdoor damage due to the sun’s rays. Check out our post on spar urethane vs. polyurethane vs. polycrylic for an in-depth guide on that topic.

UV Resistance Explained

Anti-UV chemicals, stabilizers, and additives are added to make a polyurethane finish UV resistant. These ingredients absorb UV radiation and dissipate the energy as low heat. This makes polyurethane finishes better at resisting the damaging effects of the sun’s rays in the form of ultraviolet light.

Cypress Aderondack Chairs Stained Red Oak & Spar Urethane Topcoat

Cypress Aderondack Chairs Stained Red Oak & Spar Urethane Topcoat

If you want to minimize wood surface graying and sun damage, a UV resistance polyurethane is a must. The good thing is that most polyurethane finishes for outdoor use include UV inhibitors, stabilizers, and other additives that improve their performance outdoors.

Weather Resistance Explained

Apart from UV resistance, an outdoor finish should also be able to handle weather changes. Rain, snow, high moisture, and other elements will affect the finish and wood. Polyurethane dries to a hard protective film that is water resistant. This is great for repelling water from seeping into the wood, where it can cause problems such as rotting.

For exterior wood and furniture, a water-resistant finish is desirable. Being more durable, oil-based polyurethane is better than water-based polyurethane. However, spar urethane will be better than oil-based polyurethane when outdoors.

Exterior vs. Interior Polyurethane

Polyurethane finishes come labeled interior, exterior, or both. Usually, exterior polyurethane is more flexible and has more additives that make it weather and UV resistant. Spar urethane is more durable when used outside than regular polyurethane and can last about 3-5 years when well maintained.

Minwax Helmsman Indoor/Outdoor Spar Urethane

Minwax Helmsman Indoor/Outdoor Spar Urethane

If a polyurethane finish is labeled for outdoor use, it is better to avoid using it indoors because of the chemical and VOCs present. For indoor use, water-based polyurethane is a better product. It has low VOCs compared to oil-based and spar urethane.

However, it is important to note that the greatest risk is when applying polyurethane. Once outdoor polyurethane hardens and cures, it is very safe to use unless you’re trying to eat the finish.

MWB-Team

Hello there! This is the Make Wood Better Team. Here, we share informative how-tos and guides focused on making wood better. Whether it's finishing wood, maintenance, or restoration, there's something on this website for you to learn and improve your skills.