Polyurethane is one of the most used finishes in woodworking, known for its durability, water resistance, ease of application, and its smooth, glossy looks that we all enjoy seeing.
You can comfortably finish wooden floors, furniture, tables, and any of your other woodworking projects with polyurethane to gain all the advantages that come with it and take your project to the next level.
While all of this sounds good, there is nothing more disappointing than waking up one day and seeing that the project you have finished with polyurethane is going yellow out of nowhere.
So, why is your polyurethane yellowing, and how can you fix it? Let’s find out.
Oil-based polyurethane turns yellow over time as a result of the oxidation reaction that occurs as it cures.
Contrary to popular belief, sunlight isn’t the cause behind polyurethane yellowing. In fact, it’s possible to use light to reverse the yellowing and restore the original color of the finish.
Moving on, let’s take a deeper look into what actually causes your polyurethane to go yellow, what you can do to bring it back to its original state, and how you can avoid the yellowing problem the next time.
Why Is My Polyurethane Yellowing?
Polyurethane going yellow is annoying without a doubt. While the situation isn’t all too bad for dark surfaces as it’s usually not noticeable, the yellowness becomes glaringly ugly on lighter surfaces.
While popular belief says that the yellowing is due to UV rays from the sun hitting the polyurethane, this is actually far from the truth.
All types of oil-based coating (which even includes oil-based paints) naturally yellow over time due to the oxidation reaction that takes place as they cure. What’s even more interesting is that darkness amplifies this effect considerably, whereas light (natural or artificial) prevents it from happening.
When an oil-based coating dries, molecules known as chromophores start to form, which is responsible for the yellow color we are used to seeing on polyurethane.
As long as the coating stays in the dark, the chromophores remain intact and keep giving off the unpleasant yellow color we all hate seeing.
On the other hand, natural and artificial light can break chromophores down, which causes the yellowness to slowly fade over time when you expose the coating to sunlight for extended periods.
At this point, you may think that this is purely wrong since your furniture has gone yellow despite it getting long hours of direct sunlight during the day. The fact of the matter is that windows block a large portion of UV rays, meaning that they most likely never reach the furniture that is located in the house.
If you would like to run your own experiment, you can easily observe this effect by leaving a piece of wood that you have finished with oil-based polyurethane outside and see whether the finish becomes yellow or not.
Without further ado, let’s get down to the avenues you can explore to fix your yellowing polyurethane.
How Do You Fix Yellowing Polyurethane?
Now that we have identified the cause behind polyurethane becoming yellow in detail – let’s look at what we can do to fix the issue and get rid of the ugly yellow color troubling your furniture.
Reversing the Yellowness with Light
As we have mentioned earlier, both natural and artificial light can reverse the yellowing effect and give your polyurethane finish a clear color once again.
So, technically speaking, all you have to do is expose the yellowed parts to light for prolonged amounts of time, and the yellowness would fade away over time, fixing the problem.
Unfortunately, this is most likely easier said than done for a few reasons.
- Getting direct sunlight on all the affected areas may be impossible. While you can move small items into the sunlight, large furniture and floors are out of the question, meaning that they would require artificial light.
- The strength of the artificial light source you plan on using may not be strong enough to reverse the effect or take a very long time to work, leading you to believe that it’s not working.
- Just like the polyurethane becoming yellow over long periods, reversing the yellowness will also take a considerable amount of time. While the yellowness will show signs of fading gradually, the amount of change required for you to see the difference may take quite a while.
If you are planning on using this method to try and reverse the yellowness of your polyurethane finishes, you will need a bit of luck, strong lights, and a whole lot of patience.
Sanding and Re-Finishing
While this isn’t much of a “fix” in the sense of getting rid of the yellowness, it’s the only alternative if you haven’t managed to reverse the yellowness by using light.
On the other hand, this method will certainly give you results and save you from the problem.
- Start by sanding the existing oil-based polyurethane finish off. 400 grit sandpaper is usually ideal for the job.
- Ensure that the surface is clean and free of any leftovers that may have remained after the sanding process. As this step is vital for the next finish to turn out well, be extra careful.
- Re-finish the wood with a water-based polyurethane or a completely different clear coat that won’t turn yellow over time.
Preventing Polyurethane from Turning Yellow
As the conditions that cause polyurethane to turn yellow, preventing it requires a decent amount of effort.
Here are some of our tips that can help you to prevent polyurethane from turning yellow.
- Keep the projects that you have finished with oil-based polyurethane in well-lit areas to cause the chromophores to break down.
- Apply polyurethane in thin layers. While this won’t directly prevent the polyurethane from going yellow, it will cause the yellowness to be much less and make it easier to sand the polyurethane off for a re-coat if required.
- Use water-based polyurethane. Unlike oil-based polyurethane, water-based polyurethane does not go yellow as it has a different chemical structure.
What Kind of Polyurethane Does Not Turn Yellow?
Unlike oil-based polyurethanes, water-based polyurethanes do not turn yellow.
The oxidation reaction which forms chromophores that cause polyurethane to turn yellow is something that we only observe in oil-based coatings. As water-based polyurethanes have a completely different chemical composition, yellowing is not an issue with them.
While searching for water-based or water-borne polyurethane is usually enough to find the right product, you can also see these products be marketed with labels such as non-yellowing.
As always, buy products from brands you trust, as low-quality polyurethane may cause you issues even though it’s water-based.
How Long Does It Take for Oil-Based Polyurethane to Yellow?
As the amount of time it takes for oil-based polyurethanes to yellow depends on various factors, it’s impossible to even come up with an approximation that holds any weight.
The brand of polyurethane used, the thickness of the finish, and the amount of light the finish gets are all factors that can affect the time in different ways.
As a rule of thumb, you can assume that the yellowing process starts as soon as the polyurethane dries in most cases, but it can take quite a while until the yellowness becomes noticeable.
Non-Yellowing Clear Coat Alternatives for Wood
If you had enough with the yellowing problems and would like to replace polyurethane with something else, here are some non-yellowing clear-coat alternatives you can use to finish your wood.
- Water-based varnish – Water-based varnish is a clear finish that you can use for your outdoor projects as it protects the wood against UV rays. Compared to polyurethane, varnish is harder to apply as it’s prone to cracking and peeling.
- Water-based lacquer – Water-based lacquer is a clear finish that is often preferred for its quick drying time and ease of application. That being said, it is less durable compared to polyurethane and should only be used for indoor projects.
- Tung Oil – Tung oil is a durable finish that is often preferred due to it being completely natural. While it’s an easy finish to apply, it takes a very long amount of time to dry compared to the rest.
Polyurethane turning yellow is a natural phenomenon that you can observe in all types of oil-based coatings as it’s a result of an oxidation reaction that takes place as the finish cures.
Contrary to popular belief, this phenomenon takes place in areas with no light, whereas exposure to light can reverse the effect and restore the original transparent color of the finish.
If the yellowness is becoming a big problem that you cannot contain, replacing oil-based polyurethane with a water-based one is the best solution, as water-based polyurethanes are not affected by the yellowing issue.