What’s the Difference Between Spar Urethane and Polyurethane?

Spar urethane and Polyurethane are the two wood finish products that I have been using for a very long.

Although the names are similar, there are differences between spar urethane and polyurethane that are important to know when using a finishing product.

While both are derived from oil, they also have some unique qualities that make them different enough that you will have to consider which is best for your needs.

Spar Urethane

This is a finish that is found/applied on wood products that are exposed to the elements.

Because spar urethane resists water, you’ll often find it used on wood that is submerged such as the hulls of boats and the like.

What is Polyurethane?

This is one of the most common substances used in the world.

For finishes, polyurethane usually coats many different materials along with items such as furniture.

You’ll find polyurethane in paints as it provides a shiny, smooth finish.

Although not used as a finish or coating for every product, it is certainly the most popular.

Spar Urethane vs Polyurethane: Differences

It should be noted that both products contain many similar elements.

Plus, the basic features of both are nearly the same as well. Both provide a strong, resilient finish that resists impacts, chipping, and the like.

They are also both highly popular because they are relatively inexpensive, although spar urethane does carry a higher price.

Both are generally lightweight, flexible, and strong when it comes to finishes.

It’s also why you often see polyurethane added to so many different products. But even with all the similarities, there are some noteworthy differences.

1- Difference in Application:

Both spar urethane and polyurethane are applied to surfaces in roughly the same manner.

Most will use brushes and apply even strokes for coverage.

However, polyurethane dries slower compared to spar urethane which means that it takes longer for the application to fully adhere to the surface.

Spar urethane not only dries faster but also tends to not need as many coats or layers to get the shine that is associated with the product.

2- Difference in Oil Amount:

Arguably the most noticeable difference is the oil concentration between spar urethane and polyurethane.

Spar urethane has a higher amount of oil which is why it is sometimes called “long oil varnish”.

The amount of oil in spar urethane makes it softer and more pliable compared to polyurethane which tends to be quite hard when dried.

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Spar Urethane and Polyurethane: Pros and Cons

Both products have their advantages and disadvantages.

If you have a surface to cover, it is helpful to know which will work better depending on the results that you desire.

Pros

Polyurethane:

The advantages of polyurethane start with the smooth finish it provides, especially on surfaces that are quite hard or resilient such as metal or rubber.

• Resistant to Harsh Environments
• Holds Off Oxidation
• Smooth Results

It should be noted that polyurethane is also water-resistant, durable, and can withstand heat and cold with little damage.

Spar Urethane:

As noted earlier, the main advantage of spar urethane is its resistance to the elements such as water, light, and heat.

This makes it perfect for surfaces that are outdoors.

• Low Amount of Toxic Substances
• Does Not Produce Strong Fumes
• Blocks Ultraviolet (UV) Light that Damages Surfaces

Although both substances are quite good at what they do, there are some inherent disadvantages they have as well.

Cons

Polyurethane:

Arguably the biggest drawback to polyurethane is the number of toxic substances found in the product.

It was believed for decades that once polyurethane hardened that the toxic elements were trapped.

However, recent studies show that small amounts still leak into the atmosphere.

This means that in enclosed environments such as interior rooms toxic elements can build up in the air over time.

• Takes Longer to Dry
• Difficult to Remove

Spar Urethane:

Although seemingly better in many ways than polyurethane, one of the biggest drawbacks is its inability to mix well with oil-based paints.

This is particularly true with paints that contain polyurethane.

The result is that the color of the paint will not look balanced. You may find that it does not work for you at all once it has dried.

• Strong Fumes
• Consistency of Quality
• Price

It should be noted that spar urethane is not only more expensive, but the overall quality is not consistent.

You will need to keep this in mind when deciding which product to purchase.

But it should be noted that the price difference is not too extreme depending on the brand and availability of the spar urethane.

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Spar Urethane OR Polyurethane: Which is Better for Your Project?

Depending on the surface you want to cover, it may be difficult to know which one is right for your needs.

This is where getting advice from experts such as those at your local hardware store can be helpful.

They understand the basic use of spar urethane and polyurethane and which surfaces are best suited for either product.

Plus, if you are covering a surface on an item that may be moved from indoors to outdoors from time to time, then they can provide answers for that as well.

Choosing between polyurethane and spar urethane will depend mostly on the conditions where the surface exists.

Spar urethane is better for surfaces that are exposed to the elements and particularly those that will be underwater.

It’s smooth and easy to apply but does not mix well with oil-based paints.

So, if you were planning to cover the surface with oil-based paint, then spar urethane may not be the best choice.

For interior surfaces, polyurethane is generally better. Although be mindful that it does create toxic fumes.

You will need to apply the substance in a well-ventilated area while wearing the appropriate protection, particularly a mask that covers the nose and mouth.

However, if you cannot apply polyurethane to a surface while being in a well-ventilated area, an example would be a room or basement with poor ventilation, then you should consider spar urethane as a good substitute.

You will still need to wear a mask, but it does dry faster and creates fewer fumes compared to polyurethane.  

Keep in mind that spars come with a higher oil ratio than oil-based polyurethane or regular interior varnish.

So, avoid putting polyurethane over spar urethane.

Guys at Benjamin Moore and Varathane also suggest that always sand the old finish and put the same as what you already have. This will deliver the best finish without extra effort.

Related FAQs

Is There a Difference Between Urethane and Varathane? 

Urethane is a type of polyurethane, while Varathane is a brand name for a specific type of urethane product.

Both are finishes that are applied to wood surfaces to protect them from scratches, nicks, and other types of damage.

What is the Difference Between Polyurethane and Polycrylic?

Polyurethane is a type of plastic finish that is often used as a finish for wood surfaces.

Polycrylic on the other hand is a water-based product that contains acrylic resins.

Both are used to protect the wood from damage, but polycrylic is less durable than polyurethane and will yellow over time.

Polyurethane vs. Lacquer Finish: What’s the Difference and Which is Better?

Polyurethane and lacquer both can provide a durable, protective surface that will resist scratches and stains. 

However, there are some key differences between the two finishes that should be considered when choosing which one to use.

One of the main differences between polyurethane and lacquer is the way in which they are applied. 

Polyurethane is a thicker finish that is applied with a brush or roller, while lacquer is a thinner finish that is sprayed on. 

This can make lacquer easier to apply evenly, but it also means that it dries more quickly and can be more difficult to repair if it is damaged.

Polyurethane is more resistant to scratches and stains than lacquer, but it can get yellow over time. 

Lacquer is less likely to yellow, but it is not as durable as polyurethane and can be more susceptible to damage.

Based on the above, there is no definitive answer as to which finish is better. It depends on your specific needs and preferences. 

If you are looking for a durable finish that is resistant to scratches and stains, polyurethane may be the better option. 

If you prefer a thinner finish that dries quickly and is easier to apply evenly, lacquer may be the better choice. 

Ultimately, the best finish for your project will depend on your specific needs and preferences.