Does Polyurethane Yellow?

Applying polyurethane to your flooring is a great way to preserve its natural look while adding a beautiful sheen and protecting them from abrasions and water damage. Despite what urethane manufacturers claim, given enough time, urethanes will yellow.

Luckily, there are plenty of ways to postpone this yellowing process. Thoroughly assessing your next project, properly preparing the surfaces, and choosing the correct product are all ways you can extend the life of your pristine clear coat finish.

What Is Urethane?

Urethanes are used in many different products that we use on a daily basis, like electronics, plastics, and clothing. In the construction industry, it is most commonly used in a liquid form as a sealant on exposed wood. 

Its popularity stems from its ability to protect wood from water, oil, abrasion, heat, and UV damage. Not only does it protect the wood, but it also adds a luster, helping to bring out the natural grain pattern, making it more aesthetically pleasing.

The two most common types of urethane are polyurethane and spar urethane, each having its own strengths and weaknesses.


Polyurethane is the most popular product for sealing and protecting interior wood projects. It provides a finish that protects from abrasions, heat, moisture damage, mildew, and fungus. 

Polyurethane can flex as wood expands and contracts as the seasons change, but not as well as spar urethane. 

Water-based versions are more environmentally friendly, dry faster, and produce less fumes than oil-based versions. These characteristics make it ideal for indoor projects where ample ventilation may not be possible.

Spar Urethane

Like polyurethane, spar urethane comes in both oil and water-based versions, though oil-based versions are the most common. It was originally used as a marine varnish on the spars of ships, the wooden supports that the sails of a ship were attached to. 

Compared to polyurethane, it has a higher ratio of resins and additives that make it superior at protecting against UV light. This makes spar urethane a better choice for exterior projects that need to withstand heat, snow, and UV exposure.

Does Urethane Yellow?

The short answer is yes, but there are steps you can take to postpone this process and keep your finish as clear as possible. Some of the reasons that urethane turns yellow over time are unavoidable, but using a water-based product instead of an oil-based one will definitely help slow this process, especially on lighter-color surfaces.

Spar urethane is clear when applied, but because of its higher oil content, it will turn yellow over time. Just like with polyurethane, you can avoid this by using a water-based spar urethane on lighter surfaces. The lack of oils will prevent the urethane from yellowing as fast.


Leaching is the process of the urethane absorbing particles from the surface you apply it to. If you apply urethane without thoroughly cleaning and prepping your floors, there is a very good chance it will leach into your clear layer of poly and turn it yellow.

It is important to take the extra time to strip, sand, and clean away as much of the chemicals and dirt from your floors before applying urethane to avoid premature discoloration. The cleaner the surface, the better the urethane will adhere.


Oxidation occurs when your urethane comes in contact with oxygen, and the result is a yellow discoloration. This may sound unavoidable since urethane has to come into contact with oxygen as it dries, but this isn’t entirely true.

Reusing old cans of urethane isn’t recommended, despite what the manufacturer prints on the back of the can. For example, if you have a half-full can in storage, the empty top half of the can will be occupied by oxygen. The urethane that is sitting in the can will be undergoing oxidation as long as it sits.

To avoid this, buy only as much urethane as you will need for your project so that the excess doesn’t go to waste.

If you must store excess urethane, pour it into the smallest possible container so that the least amount of oxygen will be present. Be sure to fully seal the container as well so that oxygen can’t make its way into the container and further oxidize the urethane.

Heat Exposure

High temperatures can also speed up the degradation of your urethane. Poly isn’t designed to handle exposure to heat as well as spar urethane, so choose wisely where you plan to apply it. Using poly on interior projects that aren’t near areas that produce heat, like ovens, water heaters, or direct sunlight, is the best way to avoid this.

UV Exposure

UV rays help speed up the oxidation process that causes urethane to yellow. Using poly for projects that won’t be exposed to constant direct sunlight will slow this process, leaving your urethane with a clear finish for a longer period of time. Spar urethane can handle the elements much better than poly, making it a better choice for exterior projects.

What Should You Do To Prevent Urethane From Yellowing?

  1. Use water-based urethanes instead of oil-based products on lighter-colored paints or woods. The oils in urethane will cause them to produce a yellow or amber discoloration at a much faster rate than water-based products.
  2. Use oil-based urethanes on darker woods and paints as they won’t show discoloration as easily versus lighter surfaces. They also have the added benefit of requiring less upkeep because they are thicker and more resistant to the elements.
  3. Sand in between applying each coat of urethane. You will almost always need to apply more than one coat of urethane to achieve a pristine finish, so give yourself enough time to sand and clean each coat once it has dried. This will also help prevent visible brush strokes and bubbles that may have formed.
  4. Use polyurethane for interior projects that will have less exposure to heat, UV rays, moisture, and foot traffic. This will allow for more time between maintenance and reapplying new coats.
  5. Use spar urethane for exterior projects that will constantly be exposed to heat, UV rays, and moisture. It was developed to withstand the elements much better than polyurethane, so you can expect less maintenance when using a spar product.
  6. Use a urethane with a UV blocker. Some urethanes come premixed with a UV blocker to help prevent degradation and discoloration from sunlight exposure over time.
  7. Apply thin coats of urethane. This will require applying more coats to achieve your desired finish but will reduce the chance of uneven layers.
  8. If you are reusing an old brush, make sure to clean it before use. Using mineral spirits will remove any oil-based contaminants, and dish soap will remove water-based contaminants that could discolor your finish.

What Should You Do If Your Urethane Has Already Yellowed?

Fear not, if your urethane finish is showing signs of discoloration, there are ways to fix this problem. Some will require more work than others, but a little patience will usually get your surfaces looking back to brand new.

The correct answer is going to depend on whether the yellowing is a result of oxidation, leaching, or UV & heat exposure.

If contaminants have leached from the wood into the urethane, you will have to sand off all of the existing urethane and sand off any potential impurities in the wood to prevent this from happening again.

If you reuse an old can of urethane, then it’s quite possible that it had time to oxidize in between uses. The best way to avoid this is by using a fresh can of urethane. Sand off the current layers of urethane and reapply using a brand-new product.

If your urethane has been exposed to excessive heat and UV rays over the years, you will want to sand down the discolored layers until you reach the clear untainted layers. Clean this surface and apply new layers to achieve that clear coat that you once had.

Final Thoughts

If your main concern with applying urethane is keeping its clear and clean luster, then you should opt for a water-based poly or spar product. The lack of oils will prevent yellowing or ambering over time. This is especially true when applying them to lighter-colored wood, paint, or stain.

Opt for polyurethanes on projects that won’t have to withstand constant UV exposure. Exterior projects that can’t escape UV exposure will call for a spar urethane.

Oil-based products will last the longest but will show signs of discoloration the fastest. It is best to use these products on darker-colored woods, paints, or stains.

Meet your Flooring Expert

Travis McCullough

Travis McCullough

Travis is a lifelong jack-of-all-trades in the construction industry with 20 years of experience in a variety of fields. He’s tackled flooring, carpentry, and everything in between on residential and commercial projects of all shapes and sizes.
Working independently and as part of a crew has equipped him with the know-how to not only complete a project but also teach others the finer points within most building professions. When he isn’t out hanging off of a ladder or crawling around on a roof, Travis spends his time educating people about the construction industry.

About Pro Flooring Zone

Pro Flooring Zone provides you with experts to answer your questions and reveal their insider secrets.

Recently Published Posts