Spar urethane is a type of clear wood finish that’s easy to apply, very durable, and protects outdoor furniture.
It takes 3-5 coats of spar urethane to give your wooden furniture adequate protection. The more coats of spar urethane the better the protection from water, moisture, and the sun. Proper methods of application also are to be considered when applying spray urethane to outdoor furniture.
In order to get the best finish and give your furniture long-lasting protection, it is important to apply the spar urethane correctly. Read on to learn more about how to choose the right products and how to use them.
Do I need to Sand Between Coats?
Very light sanding between coats will help the spar urethane adhere to the wood better. However, rough sanding will leave scratches and ruin the finish. Between coats, use a sanding sponge or 220 grit (or higher) sandpaper to gently rough the surface before the next coat.
Spraying vs Brushing Spar Urethane | What’s Better Coverage?
Getting good coverage is important when using spar urethane to protect outdoor furniture. You can either spray or brush the coats on, but which is best?
Brushing is fine, but it can be difficult to avoid bubbles and brush strokes because the mixture is so thick. Using the right type of brush is key, but even then, it can be a challenge to get an even finish.
Spraying is a good method for getting an even finish but you need to thin the spar urethane so it is the right consistency for spraying. Keep in mind that it is more likely to run because it is thinner, so it takes some practice to get an even finish. Even strokes and continuous movement will prevent a buildup of spar urethane, so it does not drip.
How Do You Thin Spar Urethane for Spraying with a Spray Gun?
Water-based spar urethane can be thinned with water. Adding 10-20% water will give you the right consistency for spraying.
Oil-based spar urethane needs to be thinned with mineral spirits or paint thinner, and you will need higher amounts. It takes between 25-50% volume of mineral spirits or paints thinner to give you the right consistency for spraying. The desired consistency will depend on your spraying system and some will need it to be thinner than others.
What is the Best Brush for Spar Urethane?
Choosing the right kind of brush is important when you are applying spar urethane. Use a 2-inch bristle or foam brush for the best results. Foam brushes are cheap and disposable, so you don’t have to clean them. This is a bonus with oil-based spar urethane.
A good quality brush with natural bristles, not synthetic ones, will give you a better finish when using oil-based spar urethane. However, synthetic brushes are easier to use with water-based spar urethane.
Should I Use Water-based or Oil-Based Spar Urethane?
Water-based spar urethane is easy to apply and gives you better control with detailed work. However, it is more difficult to get good coverage and it has a short drying time.
Oil-based spar urethane is thicker, so it’s more forgiving when you are working on large surfaces like armchairs or benches. Oil-based finishes also penetrate deeper into the wood for better protection against water damage.
You will notice a difference in the finish too. Oil-based tends to give a deeper color that is closer to natural wood, while water-based spar urethane can give a dull finish.
Oil-based also lasts much longer and it is less expensive, so it is the best option, in most cases. However, for furniture with intricate designs, water-based may be better as it’s easier to get an even finish.
What are the Best Spar Urethane Products for Outdoor Furniture?
Minwax Helmsman Spar Urethane 
Minwax Helmsman Spar Urethane is an excellent protective finish that is available in both water-based and oil-based options. It gives a great clear finish so it doesn’t hide the natural beauty of the wood. The UV blockers reduce the damaging effect of the sun and keep your furniture in good condition for longer.
This product can be used on its own, but it is designed as part of a 3-step Minwax treatment process, that involves prepping and staining first.
Rust-Oleum Marine Spar Varnish 
This Rust-Oleum Marine Spar varnish was originally designed for use on the above-water wooden fixtures on boats. However, it is excellent for use on outdoor furniture too. As you would expect, it is designed to be highly water-resistant and protect against salt damage too, so it will withstand the weather easily. The oil-based finish dries in roughly 2 hours and has a nice gloss texture to it.
Varathane Diamond Spar Urethane 
The Varathane Diamond Spar Urethane is a durable, water-based coating for your outdoor furniture. Unlike some other products, it doesn’t leave a yellowy finish, it dries completely clear. It is also extra fast-drying, so you can apply up to 3 coats in a single day.
The fact that it is water-based means that it is low odor and it’s very easy to clean up with water.
If you are wondering if you can treat untreated wood, check out our full article on “How to Treat Untreated Wood for Outdoor Use” here!
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.