Can You Use Spar Urethane on Cedar?

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Ensuring the durability of your cedar woodwork and furniture hinges on the application of the appropriate finish and sealer. Spar urethane stands out for its robustness across various wood types, but how does it fare with cedar?

In this comprehensive guide, I will unravel whether spar urethane is the right choice for cedar, particularly in making it resistant to weather and water.

Is it Recommended to Apply Spar Urethane Over Cedar?

The best finish to apply to cedar wood is spar urethane, brought about by its sealing functionality.

It easily hardens and cures cedar, sealing a protective coat that can tolerate even the worst weather conditions like snow, rain, sunlight, and high humidity.

When sealing outdoor furniture, you want to use a thick and harder than regular polyurethane finish to ensure a generous amount of protection on your wood, and spar urethane is your go-to. Some prefer to apply a lesser amount for a thinner finish.

How Many Coats of Tung Oil Do You Need on Cedar Wood

Two main types of spar urethane can be used on cedar. Oil-based spar urethane and water-based. Here’s how the two compare when applied:

Oil-Based Spar Urethane (When Applied on Cedar)

Water-Based Spar Urethane  (When Applied on Cedar)

Cheaper

More Expensive

Has a darker color, and the wood darkens over time 

Has a naturally clear finish

Creates thick coating

Has thinner finish

Susceptible to dents

Susceptible to scratches

Produces an amber or yellow tone

Produces a natural clear tone

Requires mineral spirit and Is difficult to clean

Tolerable with water

Lesser number of coats

Needs more layers of coating

Both water-based urethane and oil-based urethane work efficiently when applied in cedar wood, which can be a great protective coating and protection from extreme weather conditions.

Recommended Applications for Spar Urethane

Applying spar urethane on cedar is not as complicated as it seems. You can apply it on multiple cedar furniture and woodworks, such as the following:

Advantages of Using Spar Urethane

paint brush and Minwax Helmsman Indoor-Outdoor Spar Urethane

Apart from being an excellent UV blocker, there are numerous advantages of applying spar urethane on wood.

How Do You Use Spar Urethane On Wood?

Spar urethane has multiple advantages when applied to cedar wood, making it a staple when finishing cedar furniture and woodwork. Often, the greatest nemesis of wood is sunlight. On the other hand, Spar urethane has UV blockers that prevent direct sunlight from ruining the cedar wood.

The UV blockers that are components of spar urethane blocks the UV rays that come from direct sunlight, therefore protecting the wood from damage. Thus applying urethane on cedar wood assures the woodwork is protected even when placed under the sun.

Tools & Materials

Here are the tools and materials you’ll need to prepare when applying spar urethane to your cedar woodwork.

Step #1: Prep and Clean the Surface

Apply Tung On Cedar Wood Pieces

Lay the cedar wood or woodwork in an open or well-ventilated area to ensure proper air circulation for easy drying. When applying spar urethane, harsh chemicals and fumes easily evaporate with proper ventilation.

It’s a must to prepare the working area and the surface of the wood before starting the application. Remove the dust, specks of dirt, grease, and debris atop your wood by using clean rags to wipe the surface.

One of the most important steps is cleaning the debris to prevent specks of dirt and dust from getting trapped under the urethane coat, causing troubles when finishing the wood.

Applying TSP or Trisodium Phosphate is also recommended to remove dirt from the surface of the cedar. Make sure to dry the wood without cracking, after cleaning, it to make sure no residue is left.

Step #2: Sand the Wood Surface

Use 80 to 120 grit sandpaper to sand the wood surface and remove bumps, drips, and random curves usually present in bare cedar woods.

sandpapers

Sand in the wood grain’s direction and apply an even amount of pressure to ensure even sanding and avoid making unnecessary scratches.

Sanding allows the tiny, porous structures to open and easily absorb the urethane for a better finish after application. It also allows the spar urethane to stick better and prevent you from missing any spots.

After sanding the wood, clean the surface with rags to remove the sawdust and prevent it from interfering with the coating process.

Step #3: Thin You Spar Urethane (Using Mineral spirits or Denatured Alcohol)

You can achieve a smoother and thinner coat of spar urethane, making it easier to apply, by diluting the mixture with either mineral spirits or denatured alcohol. Thinning the spar urethane allows for better flow and ensures a more even application on the surface.

Spar urethane can be very thick when applied, and a thicker finish of spar urethane on cedar wood means a longer drying process. Thinning the mixture will ease absorption and help avoid splotchy and tacky surfaces. Here are a few tips to consider:

  1. Spar urethane can be applied by spraying, brushing, or wiping.
  2. To apply thin layers easily, use a sprayer, which automatically thins the spar urethane.
can of mineral spirits

Thinning the spar urethane is helpful if you apply it through a brush or a rag. Adding other solutions, such as mineral spirit and denatured alcohol, speeds up the evaporation, thinning the spar urethane. 

Step #4: Apply the First Spar Urethane Coat

Applying the first layer of spar urethane coat can be overwhelming, but there are numerous application methods you can choose from, so you can find which process works better for you. Choose from brushing, spraying, or simply wiping it.

Like applying spar urethane on painted wood, remember to apply it along the direction of the grains, regardless of what process you’re using. Another tip when applying spar urethane is to test it on scrap wood before applying it on the surface to minimize possible mistakes.

If you’re using a brush, always catch runs and brush with even strokes to avoid roughness on the surface. Hence if you’re applying with a rag, apply even pressure so you don’t miss any spots.

Also note that when applying spar urethane for the first time, the wood is more absorbent than on the next applications. Let the first coat sit for about 30 minutes, and wipe off the excess using a clean rag or lint-free cloth.

applying spar urethane to wood

Always use a quality bristle brush when applying spar urethane and a clean rag if you are wiping. When using a brush to apply spar urethane, always opt for a quality bristle brush to prevent air bubbles from appearing on the finished surface.

Step #5: Dry It Properly

Let the first layer of spar urethane dry on the cedar before reapplication. This is important to avoid tacky finishes. Depending on the coat’s thickness, the first layer can take about 24 hours to dry completely. Some take longer, such as those that use oil-based spar urethane.

Another factor is the area’s humidity. Based on the humidity level, the drying time may vary. Once the surface has dried, you will notice the mistakes you’ve made during the first application, as the areas will show right away, especially the uneven ones.

To fix this, re-apply a minimal coating to even the finish, but make sure you do this gradually to avoid unnecessary bumps on the surface. Keep the area dry before reapplication.

using hand brush

Step #6: Sand the Coat Lightly

Once the surface has dried completely, use 320-grit or 400-grit sandpaper to sand the layer and build stronger adhesion on the next layer. Only light sanding is recommended at this step as it’s only meant to prepare the surface for applying another layer of spar urethane.

Unlike what most would presume, a sanded layer has more adhesive properties and will allow the next layer of the spar to stick better. It also prevents layer separation, giving the surface a nicer and smoother finish.

Always clean the surface with a rag before applying the next coats to avoid residue from sticking onto the layers.

Step #7: Apply More Coats of Spar Urethane

I recommend applying at least three coats of spar urethane to any cedar wood to ensure a proper sealing and protective finish. Apply the same method when finishing all coats to ensure even and consistent finishes. Do not leave brush marks on the surface of the wood.

If you want a stronger finish, you can also apply up to a maximum of 5 coats but ensure that every layer is thin and dried carefully so it doesn’t blur and that the wood grains are still vivid. This also ensures there’s a beautiful sheen on your wood surface.

thinning spar urethane

Apart from protecting the wood, spar urethane also enhances the wood’s appearance and increases aesthetic properties.

Step #8: Dry and Cure the Coats

Once you’ve applied the final coat of spar urethane, ensure it cures for at least 24 hours or 1 day to at least 2 to 3 days. Curing will take time, especially for oil-based spar urethane.

However, ensuring a super hard protective coat between the outside wood and underneath is important.

How Long Can Spar Urethane Last Outdoors?

Under challenging conditions, the lifespan of spar urethane may be limited to 2-3 years. Nonetheless, with proper maintenance and regular cleaning, a single coat has the potential to endure for well over a decade. 

To uphold the wood’s lustrous properties and guarantee its protection, it is advisable to reapply spar urethane once every two years.

My Recommended Finish For Cedar Wood (Exterior Oil and Sealer)

wood Danish oil

Different finishes go well with cedar wood; here are some of the best ones you’ll find.

Spar Varnish and Spar Urethane are still the best finishes for your cedar wood exterior as they offer tougher sealing properties.

What Kind Of Spar Urethane Should You Use On Cedar?

Choosing between different spar urethane on your cedar may be overwhelming due to its number. Here are the most popular products you can use.

(How about other wood types? Can you use spar urethane on pine? Find out here!)

How Do You Protect Cedar For Outdoor Use?

Cedar wood is good for outdoor use, and the most effective way to protect it from extreme weather conditions is by applying waterproofing sealers such as varnish, tung oil, polyurethane, and the most effective spar urethane.

Quality finisher makes the cedar wood invulnerable to insect attacks, UV light, humidity, and moisture damage causing the woods to rot easily. Ensure the cedar wood is coated evenly and cured well before placing it outside.

preparing to paint cedar wood

Why Seal Cedar Wood? And Why Not

Sealing cedar wood is only a must if you are taking the wood outside and if it will stay under direct sunlight. As long as it doesn’t come in contact with soil, cedar wood can last even without treatment.

Stable

Cedar woods are warp-resistant and stable, meaning they won’t move much or grow even with water absorption. This is why you can leave it untreated, and it can last without a finisher as long as it’s not placed alongside any soil material.

Rot-Resistant

Surprisingly, red cedar, especially the western species, is rot-resistant, unlike other woods. These woods have aromatic oils that protect them from tot, bugs, and decay.

Not Fully Weatherproof

You might still want to add spar urethane on wood cedar because it’s not weatherproof. While it can resist rotting, it’s still vulnerable to weather conditions such as extreme humidity or rainwater.

Is Polyurethane An Ideal Finish to Protect Cedar Wood?

Polyurethane [1] is also a good finish to apply to cedar wood. It has a long-lasting, water-resistant film on its timber. It’s one of the stains that can prevent water and even water vapor making it a good resistant film when applied on wood.

polyurethane oil based

Thanks to its urethane composition, it has the capability to create a dense network of resin molecules, providing a protective shield to any wood surface it is applied on.

What Type Of Poly Is Good For Outdoor Cedar?

Polyurethane alone will not provide enough protection for the exterior of the wood, so if you want one that’s best for outdoor cedar, opt for a spar urethane, which has more polyurethane content.

Often called spar varnish, the spar urethane is more water resistant. It’s often used for sealing woodwork such as boats, decking, and even large marine structures.

More cedar wood guides here: 

 

Conclusion

Opting to apply spar urethane on cedar is a wise choice for those aiming to enhance the wood’s longevity and fortify it against harsh weather conditions, as well as decay and moisture. 

Just ensure you pay close attention to the application process to achieve a smooth and robust finish, safeguarding your cedar for years to come.

robert headshot

Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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