How to Keep Polyurethane from Yellowing

Image of polyurethane floor. How to Keep Polyurethane from Yellowing

Polyurethane makes an integral part of wood finishing, especially if you are looking for a lustrous appearance on your surfaces. Such surfaces could be floors, tabletops, etc. With time, it likely that your polyurethane finish might start yellowing, so here’s how to keep polyurethane from yellowing.

You can prevent the yellowing of your polyurethane finish by applying very thin layers. You need to sand between the layers using a 400 grit sandpaper. After sanding clean the surface properly, if the paint on the sanded surface is water-based, I encourage that you use a water-based polyurethane.

You realize that how you apply the polyurethane finish determines whether or not your surface will yellow. Ensure, therefore, that you get it right with your application process.

Will Oil Based Polyurethane Yellow?

Well, you can apply both oil- and water-based polyurethane to latex or acrylic paint. Even so, the oil-based polyurethane will form a yellow pigmentation to light-colored surfaces. I would recommend that you use a water-based polyurethane finish; this will help add durability without necessarily causing yellowing.

What Causes of Yellowing of Polyurethane

Polyurethane finish created by the use of an aromatic isocyanate will yellow following prolonged exposure to the sunlight due to the effects of UV light. The oxidation reaction in the core of the polymer causes the yellowing of the polyurethane finish.  

Because it is the polymer that gets oxidized in the process, you cannot extract the yellow pigmentation; ultimately, your finish will degrade. In the manufacturing of polyurethane foam, there’s a reaction between polyol, an aromatic isocyanate, plus water. The isocyanate (toluene diisocyanate), reacts with the polyol to form a urethane polymer. 

If an amine forms on any of the isocyanate groups instead of a urethane linkage, the resulting aromatic amine is susceptible to oxidation, which forms quinone. Quinones are yellow, and the continued formation of this compound will result in more yellowing.  

Considering that the quinone structure is a component of the polymer’s core (backbone), there will be discoloration of the foam before the degradation occurs. As soon as the oxidation starts to break the chemical bonds, the polyurethane will become weak and degrade.


  • The Ultra Violet light is a catalyst for the oxidation process.
  • The quinones only form from aromatic isocyanates.
  • In the events where an aliphatic isocyanate is used, the quinones do not form, and the polyurethane finish does not discolor. However, aliphatic isocyanates are not easy to process; besides, they are expensive and not commonly used in the making of flexible polyurethane foam.

Can You Put too Many Coats of Polyurethane?

Overall, 3 coats of polyurethane are enough to give you an excellent finish, anything more than that does not make any difference and is not necessary. During your application, apply thin layers of the poly finish, you need to buff additional layers such that you are in a way buffing off half of the initial layer.

What if I have dark hardwood floors? Should I worry about Yellowing?

With dark hardwood floors, you won’t notice the yellowing of the polyurethane; this does not mean that your finish is not turning yellow. Since the stain is dark, its color will prevail. You can opt for oil-based or water-based polyurethane, depending on your needs, such as appearance, smell, dry time, budget, and preferences.

Types of Polyurethane

The common types of polyurethane finishes include:

The Sheen

If you are using sheen to classify polyurethane finishes, you will find them in three types of sheen. 

High gloss polyurethane works best on the exterior or interior areas. They are sturdy, flexible, high-performing, heavy-duty, finish that cures into shiny surfaces. Mostly, they are for industrial usage. They are best at covering prepared metals, like iron and steel, galvanized steel tas well as walls, and old or newly poured concrete. 

Semi-gloss polyurethane cures to give a sheen that falls between gloss and flat. Mostly, you will find the semi-gloss polyurethane on furniture, woodwork, doors, wood floors, and kitchen cabinets. I recommend that you apply this finish using either bristle or foam brushes. Alternatively, you can use a lamb’s wool applicator. The fast-drying semi-gloss polyurethane takes just about 24 hours to dry. 

Satin polyurethane is fast-drying, lasting matte finish. It gives the wood surface a more subtle gloss with shine and a bit of color.

The Application

In this case, there’s the wipe-on poly; most people think to wipe on is the best way to apply a polyurethane finish, especially if you need a clear, thin coat. I love brushing and would recommend a brush-on method of application. Use inexpensive, disposable foam, or bristle brushes. Brushing does the best work for me, and above all, you can dump them into your liter bin once you are through using them-saving your cleaning time.


They are not as durable as their oil-based counterparts. Water-based polyurethane is a favorite of many people because it contains no smell, is generally kind to the environment, and safe to apply. This formula dries very fast, easy to clean- using water and soap. Above all, this poly product remains colorless. 

The only drawback is that this type of finish is expensive if you compare it with the roil based polyurethane.

Oil-Based or Solvent-Based

These polyurethane types contain chemicals and require that you apply them on areas with proper ventilation, unlike their water-based counterparts.

One of the most famous oil-based poly finish is the polyurethane varnish. This type of varnish is long-lasting and comes with most protective features such as resistance to moisture, heat, and other solvents. When it is still fresh, this finish is bright. However, with time and continued exposure to sunlight, this finish may start yellowing. Oil-based polyurethanes are perfect for areas that are in constant and heavy usage, like the floors.

Watch the video below on how to apply polyurethane:

Benefits of Polyurethane

Polyurethane is one of the most versatile wood finish that you can buy out there. It comes with so many benefits, most of which I have highlighted below.

  • Filling the Gap Between Rubber and Plastic

Polyurethanes can withstand increased loads, unlike the rubber, thanks to the fact that they are harder than rubber and more flexible than plastics. The flexibility of the polyurethane is the reason they are strong, which gives them the ability to resist impact.

  • They Have Resistance to Abrasion

Polyurethanes makes the perfect choice for applications in areas that experience high traffic. For many years, poly has been on the use in highly abrasive environments, thanks to their excellent ability to resist abrasion. There are no single elastomers, metals, or plastics with a better abrasion resistance compared to polyurethanes. Their resistance to abrasion is about 10 times more than any other material.

  • Oil and Chemical Resistance

Other than resistance to abrasion, polyurethane materials are very capable of resisting oil as well as additional chemicals. This feature, therefore, allows the stability in water or oil, etc. In case you need an elastomer material for subsea usage, then choosing polyurethanes would be a great move.


Yellowing of polyurethane finish occurs with time and due to prolonged exposure of the surface to the sunlight. The UV light plays a significant role in oxidation, which causes yellowing polyurethane. 

How to Keep Polyurethane from Yellowing

One very sure way of preventing the yellowing of your polyurethane finish is by the use of a high-grade waterborne polyurethane finish. Water-based polyurethane gives your wooden structure a lighter and lustrous modern look. 

At this point, I hope that you are aware of how to prevent your polyurethane finish from yellowing, which makes me believe that this article has proven useful to you. If so, share with me your opinion on the comment section below.

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.

2 thoughts on “How to Keep Polyurethane from Yellowing”

  1. Hi I just recently started doing small wood projects. I have been doing wood flags and bought a Varathane oil based poly. It has started getting darker in the can. Is that normal?? Also, can I use the water based product for flags that will be put outside?? Thanks for your help.

    1. Hello Brian,

      There are water-based formulas, meant for exterior environments. So, if you prefer water-based formulas, ensure they are for outdoor usage!

      Secondly, if your polyurethane is darkening in the can having been opened for 12 months or more, then that is not normal. It might be going bad


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