Polyurethane vs Spar Urethane

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When you are into woodworking, you will find many different types of wood finishes that we apply to wood. Different situations call for other finishes.

As a woodworker, you should know what kind of finish you need to apply to a specific type of wood and in what scenario. The choice of wood finish depends on many factors like the type of wood, the application and where you will place the wooden items.

In this post, we take a closer look at two of the most popular types of wood finishes, polyurethane and spar urethane. Let’s get into the heart of the matter!

Polyurethane Vs. Spar Urethane

Before and after pics Spar Urethane on new wooden boat seats. (Woodworkingtrade)

We use polyurethane for a smooth, blemish-free texture on wood surfaces. If the wood is going to be exposed heavily to sunlight, then we use spar urethane. This category of finish is also popularly used to apply to wood that is likely to be submerged in water. Hence, spar urethane plays a pivotal role for use on boats and all types of watercraft.

As a material, urethane is flexible and malleable. Due to this property, we use it for wooden items of different sizes and shapes, and we use it as a liquid. Polyurethane is a more rigid type of finish; therefore, we use it on straight and flat surfaces. It has an advantage over natural rubber.

Spar urethane gets its name from the fact that we apply it to the spar of a sailboat which is heavily exposed to the salty air of the sea. This makes spar urethane much more durable than regular varnish, and it lasts longer, much longer. This wood finish also contains a greater oil ratio than traditional varnish or even oil-based polyurethane.

Polyurethane and Spar Urethane: Pros and Cons

Each of these types of wood finish has its particular set of pros and cons. It is useful to know each one’s advantages and disadvantages so that you can get the best results by using a suitable finish in your woodworking projects.

Minwax 356050000 One Coat Polyurethane, Quart, Satin
One Coat Polyurethane (Image: Amazon)

Polyurethane Pros

  • It has a smooth finish, particularly on surfaces like metal or rubber.
  • Polyurethane has a high resistance to harsh environments
  • A high degree of water-resistance.
  • It is a heat-resistant as well as a cold-resistant material.

Polyurethane Cons

  • Polyurethane contains a high level of toxic substances.
  • It takes a longer time to dry than spar urethane.
  • It is difficult to remove if the surface needs to be refinished.
Rust-Oleum 250241H Ultimate Spar Urethane Water Based, Quart, Satin Finish
Rust-Oleum Spar Urethane. (Image: Amazon)

Spar Urethane Pros

  • Spar urethane has a high resistance to water, heat and light.
  • It exhibits a low level of toxicity.
  • Does not produce a high level of volatile organic compounds (VOCs)
  • It has the property of blocking Ultraviolet (UV) radiation.

Spar Urethane Cons

  • It does not mix well with oil-based paints.
  • It emits strong and noxious fumes.
  • Spar urethane can be costly, depending on the brand.
  • The quality may not be consistent.
  • It is not always readily available in the market.

How to Decide on Spar Urethane or Polyurethane for Your Project

You may find it challenging to decide, depending on the surface you want to cover, whether to use either spar urethane or polyurethane. It would be a good idea to get some advice from your local hardware store. They are the best people who can put you on the right track.

When deciding whether to use spar urethane or polyurethane for your project, you need to consider the existing conditions. If the wood will be exposed to the elements and particularly submerged underwater then, spar urethane is a suitable choice. However, if you are going to mix it with oil-based paints, you would choose another type of finish, possibly polyurethane.

If you want to apply a finish to interiors, then polyurethane is quite good. But you should be wary of the toxic fumes that it can emit.  Be mindful of the area’s ventilation, and if the place has poor ventilation, you will do better to use spar urethane in preference to polyurethane.

But even in when using polyurethane, you should still wear a mask while applying the finish. And last but not least, you should not apply polyurethane over spar urethane because of the spar’s high oil content. Due to this reason, the coating of polyurethane may not adhere well to the existing surface of spar urethane.

Safety Tips While Applying Spar Polyurethane and Polyurethane

the worker sprays a brown paint on the boards on the street

Although you always have to be careful while working on your woodworking projects, you need to be extra cautious while using urethane products. Here are some safety tips that might come in handy and that you should follow carefully:

Be Prepared for Fire

Urethanes are flammable materials and can catch fire easily. Keep a fire extinguisher and a bucket of water close by in case of such an eventuality.

Wear Proper Safety Gear and Clothing

Wear comfortable and loose clothes that allow you to move around quickly and easily so that you can work safely in your workplace. Use goggles and a mask to protect yourself from the fumes that can emit these volatile substances.

Practice Good Housekeeping

Your workplace should have adequate ventilation, and there should be sufficient provision for good passage of air to prevent suffocation. Practice good housekeeping and ensure that you don’t leave tools lying around to reduce the chances of tripping and falling. Wipe up any spills immediately without delay and store all liquids and compounds carefully in the correct place in sealed containers.


Both spar urethane and polyurethane are products that can impart a quality finish to your woodworking projects. When you use either of these two finishes, you need to be aware of the material to which you will apply the finish.

In a small constricted workspace with poor ventilation, spar polyurethane is the safest finish to use. If you have small children, older adults, or allergy sufferers in your vicinity, then again, spar urethane is a good option.

If space is not a constraint and you have sufficient ventilation, you may choose to use either spar urethane or polyurethane, depending on the type of surface to which you will apply the finish.

Initially, when you start using these materials, you may feel the need to get help. In such a case, you should approach a more experienced person, or you can visit your local hardware shop for advice.

As you continue to use these two finishes in your woodworking projects, you will be able to decide for yourself where to use spar urethane and polyurethane. Once you know how to use them correctly and in the most suitable situation, you will be able to create some fine-looking woodworking projects.

Read about the benefits of wipe-on poly here.