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 so that you will have to consider which is best for your needs.
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.
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.
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.
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.
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.
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 the toxic elements can build up in the air over time.
- Takes Longer to Dry
- Difficult to Remove
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
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.
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 and interior varnish.
So, avoid putting polyurethane over spar urethane.
Always sand the old finish and put the same as what you already have. This will deliver the best finish without extra efforts.
Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for more than a decade to serve the customers in areas such as Charleston, Mount Pleasant, Beaufort, Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques that are being implemented in his profession. You may read more about him here or get in touch with him here.
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