Spar Urethane vs. Polyurethane – What Are the Differences?

Applying Spar Urethane on a Bench

If you are looking for a resistant coating for your exterior wood or boat, you will want to consider Spar Urethane. This type of finish was originally developed for marine uses but has become popular among woodworkers and DIYers as an alternative to Polyurethane. Read on to learn more about the differences, the characteristics and pros & cons of Spar Urethane vs. Polyurethane.

Spar Urethane


Spar Urethane.
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What Is Spar Urethane?

Spar Urethane is a type of synthetic varnish designed primarily for marine use. Its purpose is providing a protective barrier against air moisture and water exposure that would otherwise, in time, damage wood. Spar Urethane became increasingly popular among DIYers as these characteristics offer good protection for doors, windows and other exterior wood installations and constructions (source).

The typical requirements for a marine varnish are to protect against fresh water, salt water, air moisture and adverse weather. Due to the requirements of boat design, Spar Urethane is also designed to be a flexible product and to adjust as the wood moves both physically and due to changes in ambient temperature and humidity.

What Is the Difference between Oil-based and Water-based Spar Urethane?

Spar Urethane is a synthetic product that is available as either a water-based or an oil-based urethane. Both types have their advantages depending on where they are to be used, reasons for use and also personal preference. The water-based product is kinder to the environment and dries faster but the oil-based varnish is generally said to be a better choice for boats and for harder wearing exterior jobs.

How Does Spar Urethane Look Like?

The product is available in different finishes, satin, semi-gloss and gloss. Urethane has also been tested to be resistant to Ultra Violet light meaning it may help to protect the wood surface from the sun’s Ultra Violet rays. Thus, it prevents the original color and grain from fading and the wood retains its original appearance for a longer time.

How does the Urethane Protect Wood?

As Urethane is known to prolong the life of the varnish base it is mixed with, the wood surface has a longer protection time before new varnish needs to be re-applied. Ultra Violet light is known to be damaging to wood if exposed for long periods but correct application of Spar Urethane will help protect the wood. One of its advantages is that it will also not crack when the wood expands and contracts naturally.

How Is Spar Urethane Applied?

Spar urethane can be applied to wood in a similar way as standard painting by using either a brush or a roller. However, when bought off the shelf, some types of Spar Urethane are a viscous liquid and if applied directly it can feel almost like painting maple syrup onto the wood surface.

Undiluted Spar Urethane can harden quickly especially if applied in sunny conditions. This tends to make the product that is not diluted far more likely to leave undesired brush stroke marks. Therefore, in most cases, some sort of thinning product is added prior to application.

A spray gun may also be used for application. However, the Spar Urethane should be thinned down approximately 20 to 30% with mineral spirits or equivalent first.

Thinning will also make it easier to brush. However, with a diluted product, extra coats may be needed to attain the desired level of protection of the wood.

Prior to application, the wood must be cleaned and smoothed free of any old flakes of varnish. Spar Urethane is a versatile synthetic varnish and designed so it may be applied over old varnish or paint.

Applying Spar Urethane by Wiping without Bubbles

Apart from brushing or spraying, there is a third application method that is quick and leaves no air bubbles or brush stroke marks that are so often encountered when applying varnish. This method is the Wiping Method (source).

To wipe Spar urethane, use 50% mineral spirits and 50% urethane, mix the two solutions for a thinner and easier to apply finish. Then, use an old rag, dip it into the diluted varnish and wipe the rag across the wooden surface that needs to be covered.

A small rag about the size of your hand should be about the most efficient size for this. But remember: if using a 50% diluted mix, double the number of coats needed to be applied to achieve the same film thickness and level of wood protection as Spar urethane that has no dilution.

Polyurethane


Polyurethane.
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What Is Polyurethane?

Polyurethane coatings are synthetic polymers, or types of plastic made from long chains of repeating molecules. The coatings are designed to be strong but flexible so the material it is applied to can flex and move naturally but without the Polyurethane coating cracking or flaking off.

What Are the Different Types of Polyurethane?

Polyurethane is available both as a water-based product and oil based.

The oil-based type is thicker but requires fewer coats than the water-based type. If you are looking for an easy-to-apply product that takes less time to dry, an oil-based Polyurethane is probably your best choice however it requires more care to apply and there is a greater chance of leaving brush strokes than the water-based product. It is made by mixing many different chemicals, many of which are unfriendly to the environment due to the emissions they create. These chemicals will darken over time so the color of the wood finish will alter slightly.

Water-based or acrylic polyurethane tends to dry faster and is less smelly which makes it a preferred choice for indoor uses.

Read our article on oil- vs water-based polyurethane where we have covered the differences in detail.

How Is Polyurethane Applied?

Prior to use, Polyurethane should be stirred well to mix the chemicals together that may have separated over time. The product should never be shaken as this will introduce air bubbles that will be visible in the final finished work.

The woodwork should first be prepared by rubbing down with sandpaper or wire wool and all debris must be removed with either a blower or vacuum cleaner before applying the Polyurethane. If the product is applied before the surface has been cleaned, debris will be visible in the end finish, spoiling the job.

The application is similar to Spar Urethane except Polyurethane takes longer to dry (note that the drying time differs significantly between water-based and oil-based poly, find the details here).

This requires more time but is also somewhat advantageous as it gives the user more time to achieve a good finish and remove any brush marks. It can be applied by brush, spray (without the need for dilution), or by using the wiping method with the aid of a small piece of rag or foam pad.

The application on flat surfaces is easier than vertical surfaces where you may experience drips or runs in the product. To avoid this, thinner coats may be used but more coats will be required in turn to achieve a similar quality finish.

Once dry, Polyurethane produces a hard finish and is resistant to many chemical liquids including alcohol. After the product has been given time to harden, it produces a semi- to full glossy polished finish that is hard to beat.

Because of its high-quality finish, Polyurethane is most suited for use indoors on furniture, staircases and wooden floorings. Water-based polyurethane is often the better choice as it dries faster. Oil-based poly requires less coats but it gives off strong odors when applied because of the chemical composition, so the work area should be vented well.

Spar Urethane vs Polyurethane – Overview of Differences and Pros and Cons

What Are the Advantages and Disadvantages of Spar Urethane?

Pros

  • Spar Urethane is designed to be resistant to Ultra Violet light. The product will only degrade minimally if exposed for long periods of time.
  • Good general exterior varnish suitable for exterior doors, furniture, sheds etc.
  • Can be applied painted or wiped onto surface. Also, can be used in a spray gun but first must be diluted to about 50%.
  • Will not darken over its lifespan.
  • Less odors produced during application.

Cons

  • It is a flammable product so is applied near a heat source such as a stove or heater, care must be taken.
  • Can be tricky to paint without pre-dilution onto a large surface as product is very viscous.
  • Can dry too quickly if applied in hot weather making it difficult to create a smooth finish without brush marks.
  • Finish may be degraded if exposed to chemicals or alcohol.
  • Can be expensive to purchase and product quality varies from manufacturer to manufacturer.


Spar Urethane.
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Polyurethane.
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What Are the Advantages and Disadvantages of Polyurethane?

Learn more about polyurethane in our detailed guidance.

Pros

  • It is not flammable so can be used safely near heat sources.
  • Dries slower so the user has more time to apply and create a smooth finish.
  • Finish is resistant to many chemicals including alcohol.
  • Cheaper than Spar Urethane.
  • Can be purchased in ready to use spray cans.

Cons

  • Polyurethane Is not suited to prolonged exposure to Ultra Violet light. If it does, it can eventually degrade and shorten the life of the coating.
  • Less environmentally friendly than Spar Urethane.
  • Takes more time to dry than Spar Urethane.
  • It will darken and discolor as the product ages on the applied wood.
  • Produces a finish that is shiny and polished looking.
  • Ventilation required if applied indoors due to chemical odors given off during the drying period.
  • Should always be stirred and never shaken to avoid air bubbles being introduced into the final finish.

Conclusion

For boats, Spar Urethane is the finish of choice in this comparison, competing by Spar Varnish with similar characteristics.

DIYers and woodworker may consider Spar Urethane as an alternative to polyurethane, lacquer, varnish or even oil finishes. The key advantages of Spar Urethane are the flexibility and UV resistance (similar to Varnish) which makes it a better choice compared to polyurethane when it comes to exterior uses.

Polyurethane may be a better choice for indoor application, thanks to its hard-protective coat. As oil-based polyurethane has a strong odor and a long drying time, the water-based type has become increasingly popular for interior uses, e.g. when (re)finishing hardwood floors.

If you are looking for a durable and easy-to-apply wood finish, consider also lacquer, paint and varnish (read our comprehensive comparison). For furniture, Danish oil and other types of oil may also be good alternatives.

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