Can You Use Spar Urethane on Pine? Will it Last Outside?

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Spar urethane is a top-tier sealant renowned for its superior UV protection and resistance to moisture, making it a prime choice for outdoor wood finishes. But the question is its compatibility with pine wood, specifically whether it can safeguard pine furniture in outdoor settings and prolong its pristine condition.

So, in this article, let me explain the efficacy of spar urethane as a protective coating for pine, and provide you insights on its durability and the anticipated longevity of the treatment on pine furnishings exposed to the elements.

What Is Spar Urethane and Its Types?

Spar urethane combines synthetic urethane resins and natural ingredients, mainly drying oils, natural resins, and solvents. It is an altered waterproof sealer formula of spar varnish. This formulation makes spar urethane as one of the versatile exterior paints. 

You can apply spar urethane on wood, waterproofing it with its highly water-resistant components. It protects wood against mildew and moisture-related damage, and UV damage, preventing it from splitting or discoloring.

You can use spar urethane on exterior and interior wood surfaces, offering a water- and rot-resistant coating. Spar urethane contains resin that, when hardened, turns into a highly durable plastic material that is excellent for outdoors. 

paint brush and Minwax Helmsman Indoor-Outdoor Spar Urethane

Resins in spar urethane offer elasticity and are water-resistant because of the combined organic compounds with a chemical bond. Therefore, spar urethane adds a protective coat that is decay and rot-resistant.

However, spar urethane can also result in a slight yellowish color which is unsightly for indoor furniture. But water-based options offer clear coatings which you can use indoors.

You can use either oil- or water-based spar urethane, depending on your desired finish. When it comes to drying, water-based spar urethane dries faster than oil-based. But oil-based spar urethane weather- and water-resistant capability lasts longer than water-based.

In terms of toxicity levels, water-based spar urethane is safer than oil-based and environmentally friendly, making it ideal for countertops and shower seats.

Spar Urethane vs Polyurethane: Main Differences

However, spar urethane is not a polyurethane sealer, depending on the physical properties and material. But between Spar Urethane vs Polyurethane, both offer waterproof wood surfaces.

polyurethane oil based

Polyurethane has higher resins than oil, resulting in a more brittle, harder-dried finish. Its finish may crack or chip over time if prolonged exposure outdoors. Therefore, it is ideal for interior wood surfaces.

Meanwhile, spar urethane contains higher oil than resins, making the dried finish more flexible than poly. The dried finish of spar urethane can contract and expand despite temperature changes.

You can use it outdoors, particularly for wood surfaces constantly exposed to extreme heat and water elements. It has elasticity and moisture- and water-resistant ability, making it ideal for pine wood furniture and exterior wood works.

Is Spar Urethane Ideal to Use on Pine?

Pine is a softwood with wood grain that is wide and open, making water absorption faster than any wood. It has become popular in woodworking because it’s lightweight and has beautiful wood patterns.

With these properties, using spar urethane on your pine wood is ideal. Once exposed to air, spar urethane polymerizes on the surface, creating a barrier from outside moisture. 

It is a smooth, flexible, hard finish, and the excess spar urethane remains on the pine as a protective coating.

Ponderosa Pine wood

Spar urethane isolates and separates pine from UV damage and water and moisture damage, preventing fading and discoloration and offering a rot-resistant wood. It also improves the sheen of pine, highlighting the natural color when using water-based spar urethane.

You can apply different variants of spar urethane sheens, such as satin, semi-gloss, and gloss. Ensure you apply additional coats using spar urethane because it has a thicker consistency than poly. 

You can have different finish results when using oil- and water-based spar urethane. Here is a comparison of the two types of spar urethane.

Oil-Based Spar Urethane Water-Based Spar Urethane
Less expensiveExpensive
Less flexibleMore Flexible
Less durableCan withstand dents
Slightly yellow tone finishNatural clear finish
Thick coatsThin coats
Slightly darkens the pineNo darkening of the surface
Longer curing and drying timeQuickly dry and shorter curing time
Use mineral spirits to thinUse denatured alcohol to thin
Use mineral spirits to cleanEasy to clean

Knowing the difference between oil- and water-based spar urethane, you can identify its functionality on your pine wood. But both types of spar urethane offer a protective coating for pine wood from harsh environmental elements.

Will Spar Urethane Make Pine Tougher And Durable?

staining pine

You can apply spar urethane to make pine more durable but not tougher like a wood hardener could do. Spar urethane has limitations in reinforcing timber, but the protective coat is more flexible than polyurethane, a urethane-based sealer. 

Applying spar urethane on the entire surface adds flexibility, making scratches and scrapes and keeping the moisture out. Pine is a softwood prone to wood rot and decay if not treated with sealants like spar urethane.

Benefits Of Spar Urethane Application On Pine

Another benefit of using spar urethane on pine is protection against UV damage. You can also rely on spar urethane under extreme weather conditions such as snow, rain, and high humidity [1]. This also includes constant shifting of temperature.

Your pine furniture could also benefit from spar urethane for increased durability and better protection from warping, cracks, and insect attacks. Its flexible finishing coat gives pine furniture improvement on wood sheen. 

Eastern White Pine

Its workability and food-safe when cured make spar urethane one of the best sealers for pine furniture and other outdoor woodwork.

Where to Use Spar Urethane

Because pine is a versatile softwood, there are many pine woodworks where spar urethane application is needed. 

You can apply spar urethane finish on pine wood furniture, carvings, floorings, fences, patio furniture, cabinets, doors, and window frames. You could also have spar urethane coatings for other indoor and outdoor pine wood projects.

Can Spar Urethane Seal Pine Wood Knots?

You can use spar urethane for knotty pine wood projects, which tend to bleed and discolor the stains or paints of your wood. With the thicker consistency of spar urethane, it seals the knots, blocking the sap from the tree.

linseed oil on pine wood

However, it is recommended that you use a wood knot blocker or a primer that is more suitable for knotty wood, such as pine. The knot blocker fully seals the knots, preventing the bleeding of the sapwood. 

Spar Urethane Application on Pine: Tools & Steps

Tools & Materials

Step #1: Prep and Clean the Surface

First, you must find a well-ventilated place. This will help spar urethane dry faster and provide fresh air circulation to prevent inhaling harsh chemicals and fumes. Wear a face mask and gloves before you apply spar urethane.  

preparing work area

Then, using a clean rag with trisodium phosphate (TSP), remove the tougher grease, stains, and other residues on the surface. But for dirt, dust, and debris, you can remove it using soap and water can remove dirt, dust, and debris. 

Ensure the wipe is damp and not wet so pinewood won’t absorb too much water. You must wipe every inch of the wood, particularly areas that are hard to reach. This will prevent trapped impurities between the pine wood and spar urethane, resulting in a blotchy finish.

Once you remove the impurities, wipe them with pure water to remove all traces of the cleaning solution. After cleaning it, let the pine wood dry completely before sanding.

Step #2: Sanding Pine Wood

Sanding the pine wood surface allows for even and consistent absorption. It also eliminates drips, dumps, and roughness of the surface. 

To have a flat and smooth outdoor pine wood, use 120-grit sandpaper. This will remove raised spots and wood fibers. If your pinewood has an old finish, use a chemical stripper and scrape it before sanding. 

sanding wood

Then switch to 180-grit sandpaper to smoothen the pine wood and make the surface more even. Use a sanding block to maintain correct pressure while sanding along the direction of the wood grain. You can use an orbital sanding tool for larger surfaces like decks and floors.

Check the surface of your pine wood to see if there are any obstacles and uneven texture. Then you can collect the sand dust using a tack cloth or a soft brush, but a vacuum or dust collection pine system is recommended. 

Finally, use a damp, lint-free cloth to wipe off the remaining dust, then let the wood dry for at least two hours. 

Step #3: Thinning Spar Urethane

After cleaning the surface, you can apply spar urethane, but you must thin it. Thinning spar urethane makes your application easier and makes the coating dry quicker. 

Spar urethane comes in a thick consistency, and when applied without thinning, the drying time could take weeks, and the finish may form blotches. Therefore, you must mix spar urethane with thinning solvent for a thin coat.

holding paint brush

Get your spar urethane and mixing container. The mixture ratio is four parts of spar urethane and one part of mineral spirit for oil-based and denatured alcohol for water-based sealants.

The thinning solvents contain high evaporative chemicals, so mixing them can improve spar urethane evaporation, resulting in thin coats. 

Step #4: Applying the First Coat

With your thinned spar urethane, use a paintbrush to apply the coat and ensure the application is along the direction of the wood grain. Put enough and consistent pressure when applying the first coat of spar urethane. This will prevent from splotchy and blotchy finish. 

Depending on your spar urethane, apply it with paint spray or wipe it on the pine wood. But avoid using synthetic brushes. Instead, use a bristle brush. Then, remove the excess spar urethane using a clean rag. 

At this time, you will see that the first coat will be quickly absorbed in the wood. But you need to let it dry and saturate for at least 30 minutes.

apply spar urethane

If you see some air bubbles, this results from shaking the spar urethane container. Therefore, keep the container steady before your application to prevent bubbles from appearing. 

Step #5: Dry the First Coat and Sand the Surface Lightly

Let the first coat dry for at least 12 to 24 hours. Depending on the humidity and temperature, your waiting time could be longer than expected. Some spar urethane coatings take days to dry. 

Also, when drying the coat, place the pine wood in a dust-free environment. Then, if it is completely dry, lightly sand the surface using finer sandpaper. This will give an excellent surface for the next layer of spar urethane to stick. 

Then remove the dust using a clean rag before checking for missing spots on your pine wood surface. You can cover these spots by applying the next coat of spar urethane.

Step #6: Applying More Coats

You can apply three to five coats of spar urethane on your pine wood furniture. On each application, ensure each coat is completely dry and lightly sanded. You should also apply the urethane along the wood grain’s direction for a flat and smoother finish. 

Minwax Helmsman Indoor-Outdoor Spar Urethane

Otherwise, you could have a tacky surface with brush marks when applied across the pine wood, putting uneven pressure on the applicator. 

If you are on your finish coat application, sanding is unnecessary. The coats of spar urethane serve as a protective layer and improve the sheen and appearance of your pine wood furniture.

Step #7: Drying and Curing the Coats

High humidity and weather conditions affect spar urethane drying and curing time on a wood surface. Typically, spar urethane on pine wood surfaces needs at least 24 hours to dry and two to three days to cure. But the maximum days to fully cure is five days to one week.

The drying and curing time of the coats is worth waiting for because spar urethane offers a dried hard finish that protects your pine wood. 

Without proper sealants, pine furniture and wood works are susceptible to wood rot and decay when prolonged exposure to harsh elements. 

pine wood furniture

After your pine wood furniture coated with spar urethane has been completely cured, you can employ it outdoors. 

How Long Does Spar Urethane Last Outdoors?

Spar urethane on your wood furniture could last at least two to three years if consistently exposed to extreme weather conditions. After two to three years, the coatings will peel off, making your pine wood discolor and fade. 

However, with regular maintenance and proper care, spar urethane could last outside for almost a decade. Restore the finish and apply thin coats of spar urethane once a year. This will improve the color and prevent the yellowish tint on your pine wood.

For spar urethane in a sealed container and stored in a dust-free and dry area, it can last for five years at the maximum. But after you open it, spar urethane will start to go bad in six months to one year. 

Ways to Protect Pine For Outdoor Use

You can protect pine wood outdoors using sealants such as spar urethane, polyurethane, spar varnish, lacquer, and tung oil. A quality sealant offers a finish with UV blockers and waterproofing properties against water and humidity damage. 

tung oil on pine wood finishes

It includes a protective coat for rot-resistant. Some sealants accentuate the wood grain pattern and improve the natural sheen of your pine wood. But ensure the wooden surfaces are clean and sanded correctly before application. 

(But how to treat pine wood for outdoor use? Read more here!)

Should You Weatherproof Pine? How?

Weatherproofing pine wood can be achieved by applying quality sealants such as spar urethane, polyurethane, and spar varnish. Pine wood can tolerate extreme weather conditions and UV damage with three to five coats. 

Here are the basic steps to weatherproofing pine wood surfaces using poly, paint, and epoxy finish. 

Using Polyurethane

Step #1: Clean the outdoor pine wood surface using a clean cloth with TSP to remove grease. You can also use soap and water for simple dirt and debris. After cleaning the surface, wipe it with a damp clean rag to eliminate the residue of the cleaning solution and let it dry.

Step #2: Sand the pine surface using 180-grit sandpaper by following the direction of the wood grain. Then remove all the sand dust using a tack cloth, vacuum, or brush. The sanded surface enables a good adhesion base for your sealants.

sanding surface of MDF board

Step #3: After sanding the wood surface, get your polyurethane and apply the first coat using a paintbrush or sprayer. Brush along the direction of the wood grain, and do not use a roller because it might leave bubbles during application. 

Step #4: Let the first coat dry before lightly sanding using 220-grit sandpaper. The coarser sandpaper allows the next coat of poly to stick properly. This will result in a more durable finish.

Step #5: Wipe the surface using a cotton cloth to remove the sand dust before applying your subsequent coats. 

The standard layers of oil-finish polyurethane are two coats, while water-based are approximately three to five coats. 

Using Paint

Step #1: Before painting your pine wood, remove the old finish using a chemical stripper or sanding it with coarser sandpaper. Bare wood is ideal for paint finish, so ensure no residue of any materials are trapped during application.

applying soy gel stripper

Step #2: Clean the surface using a clean cotton cloth and TSP solution after removing the old finish. You can use a clean cloth with a mixture of soap dish and water solution if the surface has minimal dirt and grease. 

Step #3: Apply wood filler to spots and cracks on the wood. Filling cracks must be precise for a flat surface and consistently absorb paint. This is the same procedure as other woods. 

Step #4: Sand the surface using 180-grit sandpaper to add texture to the primer. Follow the wood grain direction when sanding and remove the sand dust using a damp or tack cloth. Ensure no sand dust or particles on the surface before painting the wood.

Step #5: Apply two coats of an oil-based primer or shellac to give your pine wood an excellent stain-blocking, particularly when using knotty pine wood. This will prevent the knots and tannins on your wood from bleeding and discoloring the surface.

preparing KILZ Original Primer

Step #6: Apply two layers with your desired color and gloss after sanding and filling the gaps and cracks. You must apply thin coats to avoid a wrinkled finish and longer drying time.

Using Epoxy

Step #1: Clean the pine wood surface using 180-grit sandpaper. This will make the surface level for your epoxy application.

Step #2: Follow the instructions for your epoxy product while measuring it using disposable cups. Depending on your epoxy, it requires different ratios of resin and hardener as long as you prevent having a sticky epoxy.

Step #3: Combine the epoxies in a single cup and stir them using a wooden or plastic rod. Gently stir the epoxy in one direction for at least three to five minutes. This will ensure the epoxies are well-combined and prevent the formation of any air bubbles.

Step #4: Pour the epoxy immediately into the middle of your pine wood surface, or you can spread the epoxy across the wood. 

Step #5: Use a foam brush to spread the product back and forth motions. Once you spread it evenly over the pinewood surface, wipe off any excess using the foam brush.

using foam brush

Step #6: If you sew a few air bubbles on your epoxy, use a heat gun, hair dryer to eliminate the bubbles. Hold the heat source at least six inches from the epoxy while slowly moving it back and forth. This will pop the bubbled areas.

Step #7: Let it harden for at least four hours before applying the second coat of epoxy. Use a spatula instead of a foam brush to spread epoxy on the surface for your next coat. For a smoother and even finish, use a foam brush.

Step #8: After applying the final coat, let your epoxy dry for at least 24 to 36 hours to ensure a hard-dried finish. But give it at least seven days for the curing time and excellent weatherproofing of your pine wood.

Best Finish for Pine Wood

Wood finishes for pine wood surfaces offer different results in terms of durability, workability, repairability, and aesthetic appearance. What I have here are the wood finishes that you can use over your pine furniture and woodwork.

True Oils

Tung and linseed oils are excellent choices for finishing pine wood, given their status as natural drying oils. They tend to outperform synthetic or blended oils that may include non-drying or semi-drying components, offering a superior finish.

refined linseed oil

Varnishes

Varnishes are a superior choice for finishing pine wood, thanks to their composition of synthetic resins and oils, which yield a hard and long-lasting surface. 

Common oils incorporated in varnish formulations include linseed, tung, soybean, and safflower, all of which contribute to the finish’s robustness. Because of synthetic resin and a combination of oils, it offers different finishes.

Other types depend on the percentage of oil than resin. It can have long, medium, and short oil compositions. The higher oil content is considered long oil. This is more elastic and softer. Marine and exterior varnishes are among the products of long-oil varnishes.

preparing TotalBoat Gleam Marine Spar Varnish

The varnish with equal oil and resin is called medium oil. This can protect your pine wood surfaces but will not withstand prolonged exposure to extreme weather conditions. 

The lowest percentage of oil with higher resins is the short-oil varnish. This is the opposite of long oil because it has weak elasticity and requires extremely high temperatures to dry. 

Long and medium oil varnishes are used for interior wood surfaces. In comparison, short oil is ideal for industrial use. 

Oil-Varnish Blends

Other best oil finishes for pine wood are the oil-varnish blends. A few of these blends you can use are Danish and teak oil. These blends offer a good finish using quality and easy application because of true oils.

Shellacs

Shellacs is considered the best finish for pine wood, but it works best indoors. You can have shellac pre-mixed or in flake form natural resin. 

Zinsser shellac

Depending on your application, you can mix it with denatured alcohol. It can be quickly dissolved if mixed with chemical solvents after it is dried. 

Lacquers

Lacquers is considered one of the best wood finishes because it has many uses in woodwork. Using lacquers can add richness and depth while giving durability to pine wood. The most common types of lacquer are nitrocellulose lacquer and acrylic lacquer. 

Top Spar Urethane Products for Pine

Here is the list of the top spar urethane products for pine wood furniture.

Minwax Water-Based Helmsman Spar Urethane

You can use these top recommended spar urethane which does not yellow and offers excellent protective layers against water and UV damage. These products also build a coat that serves as rot-resistant, preventing decay.

Best Exterior Oils For Pine Wood

Pinewood requires exterior oils to preserve and improve its durability and is water-resistant. With proper application of exterior oils, most woods, including pine, will have less maintenance yet remains in good condition.

Danish Oil

Danish oil combines tung or boiled linseed oil, varnish, synthetic resins, and mineral spirits. Depending on the manufacturer, the formula of the mixture varies on its purpose as exterior oil. 

But Danish oil is considered one of the best exterior oils because it offers excellent weather and water resistance. It is also food-safe once fully cured, so you can use it as coatings for your kitchen tops and picnic tables.

Tung Oil

Tung oil, which often includes food-safe additives upon drying, is renowned for providing durability and flexibility when applied to pine wood. It’s hailed as one of the premier choices for exterior oil finishes due to its status as one of the most robust traditional oils available.

sandpaper and tung oil

Teak Oil

Teak oil, a mixture that typically includes tung oil, boiled linseed oil, varnish, and petroleum naphtha, is favored by many woodworkers for application on various woods, including both hard and soft types such as pine. 

Its relatively thin consistency makes it well-suited for penetrating porous wood surfaces.

Will Spar Urethane Turn Yellow On Pine Over Time?

Spar urethane can turn your pine wood surface yellow over time if you use oil-based formulas. Spar urethane contains a yellow tint, given its high resin formula. You can observe the subtle golden hue of oil-based spar urethane. 

When applied to pine wood, the yellow tint deepens slightly, making an amber finish. If the coatings become yellow, you must reapply the wood finish because this color is a sign that the finish is wearing off. 

Spar urethane finish on the wood is recommended to replenish once a year for sufficient protection of pine wood.

What Is the Shelf-life of Unopened Spar Urethane?

Unopened spar urethane finish can be stored for at least ten years, but ensure you keep it in a cool and dry place. The storage area should be away from direct sunlight, and you should set the temperature at a normal or controlled temperature.  

Rust-Oleum 9341 Ultimate Spar Urethane Oil Based, Quart, Satin

Upon opening a container of spar urethane or if you’re storing leftovers, anticipate that the product may start to deteriorate in quality within six to twelve months. Spar urethane contains resins that are susceptible to hardening when exposed to air.

Conclusion

Utilizing spar urethane to coat pine furniture for outdoor use significantly extends the lifespan of the wood. This sealant offers unparalleled UV protection and moisture resistance, outperforming many other products on the market.

With the application methods I’ve provided, you’ll be equipped to preserve the quality of your pine wood items on your own, eliminating the need for professional intervention.

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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