Spar urethane is an excellent clear coat for protecting wood, especially for exterior applications. It’s incredibly durable and protects against moisture and UV damage. But you need to be careful when applying it because there is a serious fire risk. This article will tell you everything you need to know about the flammability of spar urethane and how to use it safely.
Is Oil-Based Spar Urethane Flammable?
Yes, oil-based spar urethane is flammable and should be handled with care. Spar urethane has a flash point of 38 degrees Celsius (101 degrees Fahrenheit), which means that it will set on fire when exposed to an ignition source if it is above that temperature. So, oil-based spar urethane is very volatile at temperatures slightly above room temperature.
Is Water-Based Spar Urethane Flammable? 
Water-based spar urethane is a safer option than oil-based products because it is not as combustible. It will still catch fire, but it is nowhere near as volatile. If you are especially concerned about fire hazards, water-based products are the way to go.
Will Spar Urethane Catch on Fire when Exposed to a Flame?
Oil-based spar urethane will easily catch on fire when exposed to a flame. Once it reaches 38 degrees and starts giving off vapors, they will ignite immediately and it will burn very easily. Even if the overall temperature of the spar urethane is lower than the flash point, it won’t take long for the flames to heat it and set it alight.
Water-based spar urethane is less flammable, but will still catch fire when exposed to flames. Although it will take longer, it will still burn quite easily.
Can Spar Urethane Catch on Fire from a Spark?
Yes, oil-based spar urethane can easily go up if a spark catches it. Once the temperature reaches the flash point, spar urethane only needs a tiny push, like a spark, to set on fire. Water-based spar urethane is less likely to catch on fire from a spark because it is not as flammable. However, if it is kept in the wrong conditions and the temperature is allowed to increase too much, a spark may be enough to set it off. So, spar urethane should be kept away from any obvious ignition sources, and electrical equipment that could create sparks.
Can Spar Urethane Catch Fire from High Temperatures?
Spar urethane has an autoignition temperature of around 230 degrees Celsius (450 Fahrenheit). That means it will spontaneously combust without an ignition source if it reaches this temperature. So, if you store it near a heat source and it doesn’t have enough ventilation, there is a chance that it could burst into flames. A sealed can of spar urethane will also give off vapors as it heats up. As these vapors build, the risk of fire or even explosion increases.
If I Leave Spar Urethane in the Sun Can it Catch on Fire?
Leaving spar urethane in the sun, especially in a sealed container, is one of the most common mistakes you can make. The temperature will quickly increase on a hot day and, if left unattended, it could potentially catch fire. The increased temperature also means that it is a lot easier for the spar urethane to catch fire when exposed to an ignition source.
What is the Flash Point of Spar Urethane?
Spar urethane has a flash point of 38 degrees Celsius (101 degrees Fahrenheit). For a substance to be classified as flammable, it must have a flash point of 93 degrees Celsius (199.4 degrees Fahrenheit) or lower. So, spar urethane is incredibly flammable, especially oil-based spar urethane.
Is Spar Urethane Flammable when Dry?
When spar urethane dries, the flash point changes and it becomes less flammable, but it can still catch fire quite easily. However, the flash point and auto ignition temperature of spar urethane is higher than that of wood. So, if you use spar urethane to coat a piece of wooden furniture, for example, it won’t make it any more flammable once it has fully dried and cured.
What’s the Safest Way to Dispose of Spar Urethane?
Oil-based Spar urethane is considered a hazardous material and should not be poured down the drain or thrown into the garbage. You can actually dispose of excess spar urethane using cat litter. Simply mix some litter into the container with the spar urethane so it soaks it up and clumps together. Let it dry properly and you can then put it in the trash.
However, if you don’t want to dispose of it using cat litter, you should get in touch with a local recycling center with the capacity to handle hazardous materials. They will be able to dispose of it safely without causing any damage to the environment or creating a fire hazard.
Are Rags or Brushes with Spar Urethane on them Flammable?
Yes, while the spar urethane is still wet, used rags and brushes are a fire hazard. If they are left in the sun where they can reach high temperatures, there is a big danger of fires. The rags will burn very quickly when soaked in spar urethane, and if you put them into the trash before they are properly dried, it can be very dangerous.
Put old rags out to dry on a non-flammable surface, like concrete, and keep an eye on them. Separate them to reduce the risk of spontaneous combustion. Alternatively, you can put them into a sealed metal container with some water to prevent fires until you are ready to get rid of them.
How Should You Store Spar Urethane?
When storing spar urethane, it is important to keep the temperature stable. Always store it away from direct sunlight in a well-ventilated area to stop fumes from building up. Try to keep it away from other flammable materials as well so you can limit the spread of fire.
If you are worried about the spar urethane going bad, you can put a thin layer of mineral spirits on the surface before you close the lid. This will stop it from turning into a thick gel, and you can simply stir it together and use it again when you are ready.
How Do You Safely Put Out a Spar Urethane Fire?
Spar urethane fires are incredibly dangerous because it burns quickly and will spread easily if not controlled. Water is not very effective at putting it out spar urethane fires, so use a dry chemical, foam, or carbon dioxide fire extinguisher instead. The fumes are incredibly toxic too, so do not approach the fire and call the fire department if it gets out of control.
Final Thoughts
Oil-based spar urethane is often preferred because it gives a nicer finish, but it’s incredibly flammable, so you need to take extra precautions. Water-based products may be a better alternative but if you do decide to use oil-based, make sure that you store it properly, apply it in a well-ventilated area away from ignition sources, and clean up properly afterward.
Read relatable flammable articles:
- Is Wood Stain Flammable?
- Is Finishing Wax Flammable?
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- Is Linseed Oil Flammable?
- Is Teak Oil Flammable?
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.