Polyurethane is a durable finish that lasts long on the material. But over time, it starts to yellow. So how do you fix this?
To fix yellow polyurethane, you can scuff the top layer with ultra fine-grit sandpaper (220-grit). The sandpaper will help to remove the tint and lighten the finish.
However, in most cases, using ultra-fine sandpaper to remove the yellow tint won’t work. So the best way to fix it is to strip the finish with an alcohol-based solvent or chemical stripper, and re-apply the sealant.
Why Does Polyurethane Turn Yellow?
Polyurethane turns yellow because of the oils in the top coat and the chemical reactions in the paint coating, polymer degradation, and UV exposure.
It turns yellow as the top coat pulls and absorbs oils, tannins, and dye from the material used on. It also changes its color over time due to usage and dust accumulation. Oil-based polyurethane tends to turn yellow more often.
Polyurethane is a very strong top coat. Its strength is due to the formula as it contains many additives including resins and plastics. Over time, a chemical reaction can occur between these compounds leading to discoloration or yellowing of the coating.
The yellowish tint is also caused by an over-accumulation of synthetic oils. Over time, the oils in the coating can turn the clear coat yellow.
UV exposure is another reason, it mostly affects indoor polyurethane that doesn’t have UV protection. This causes the UV rays to penetrate the coating and increase the heat between the coats. Over time, the over-exposure to UV rays can cause the clear coat to turn yellowish or brown.
How To Fix Yellow Polyurethane?
There is no quick fix to get rid of yellow polyurethane. This is because when it yellows, it’s not just the top coat that is affected, the undercoat will also be affected. So whatever fix you try or use on the top layer, it won’t work on the undercoat. So technically, the tint will still be present.
1. Use Mineral Spirits
You can scrub the yellowed polyurethane with mineral spirits and a soft sponge. This can help get rid of most of the tint. The mineral spirits will dissolve and remove grease or deposits on the coating.
You can also try scuffing the top layer off to remove most of the tint. This works better for polyurethane that has yellowed due to UV exposure. In this case, the top coat is usually the most affected so sanding it can lighten the finish and get rid of it.
2. Use Light To Fade The Yellow Tint
As soon as oil-based polyurethane starts to dry, an atom named “chromophores” start to form. This atom gives the sealant that yellow tint. If the coating is placed in dark places without having access to UV light, this atom will start to give the coating that yellow tint.
However, if you place the coating outside, the tint will start to disappear after a while. The coating should be outside for this to happen and not near windows because windows block UV light. However, this won’t preventit from turning yellow, it will happen eventually, but this method will slow it down.
If nothing works, you must strip the finish with a chemical paint stripper, seal the material with a stain-blocking primer, and apply a fresh coat.
Related Read: How To Fix Sticky Polyurethane?
Water-based polyurethane rarely turns yellow over time because it has a very low level of oils in the coating. It has a low level of VOCs, paint chemicals, and solvents which means the paint coating is less likely to change its color.
However, water-based polyurethane can yellow over time based on the absorbent qualities of the paint coating. It can pull or absorb oils, tannins, and stains from the wood leading to the coating changing its color.
Asides from that, cheap and low-quality water-based polyurethane products tend to change its color over time. However, this takes a while before it happens, usually over 3 years.
How To Prevent It?
The following tips will help to prevent your coating from changing its color:
- Ensure the material to be painted is dry.
- Apply a stain-blocking primer like shellac before applying the sealant.
- Use exterior polyurethane because the top coat is designed with UV blockers.
- Use sealants that contain silicone compounds like a rust-oleum sealant.
- You can also try sealing the finish with a coat of UV-resistant sealant.
Can You Paint Over Yellowed Polyurethane?
You can paint over yellowed polyurethane. But to do so, you need to sand the sealant to remove the top glossy layer first. Also, you’ll have to seal the sealant with a stain-blocking primer like shellac.
However, it’s better not to paint over it. If the sealant is yellow, it’s time to strip it and apply a new finish. This will produce a better finish and reduce the weight and number of coats on the material underneath.
If you paint over it, you should use an exterior sealant. These sealants often contain silicone compounds that prevent heat or UV rays from changing the coat color. A good choice is a rust oleum finish.
If the sealant turns yellow, this means it’s time for it to go. If you take the risk and paint over it, the whole finish can come off after a while.
Related Read: Can You Apply Polyurethane Over Polyurethane?
Types of Polyurethane That Don’t Yellow
Water-based polyurethane is known to yellow less because there is a lower level of chemicals in the formula. Also, it uses water as its solvent, so there are no oily deposits or residues that can change the color of the coating.
Water-based polyurethane is also less prone to weathering effects. UV exposure and heat will not make the water-based polyurethane change color as it would on oil-based polyurethane.
Here are the brands you should use:
- Bona Traffic HD water-based polyurethane
- Loba WS 2K Supra
- Pallmann’s Pall-X 96.
Will Polyurethane Turn White Paint To Yellow?
Polyurethane will not turn white paint to yellow. This is because polyurethane is a clear coat and as such isn’t designed with pigments that will change the paint colors.
As a clear coat, polyurethane is designed to reveal the color of the undercoat or paint underneath. So if you apply it over white paint, the paint will remain white, only that it will have a glossy clear film over it.
However, polyurethane can make white paint have a yellowish appearance over time. This is more common with oil-based polyurethane. When this happens, it’s not the white paint underneath that changed the color. Rather, it’s the sealant that begins to change its color.
Since the clear coat is changing its color due to polymer degradation or UV exposure, it can make the white paint underneath change its color too. But, if you remove the sealant, the paint underneath will have the same color.
Related Read: How To Thin Polyurethane?
Things To Know
Oil-based varathane polyurethane does yellow. However, its water-based type tends to yellow less.
Wipe on Poly
All oil-based polyurethane will yellow over time, including wipe-on poly. However, some types tend to last longer before changing their color. So, wipe-on poly takes longer to change its color, and remains clear for several years.
The time it takes oil-based polyurethane to change its color depends on a lot of factors. The thickness of the coat applied, and how much the coating gets exposed to UV lights, all determine the time it takes the sealant to change its color.
As a rule of thumb, it can take oil-based polyurethane between 6 months and 2 years to develop a yellow tint. On the other hand, water-based poly remains clear for 1-3 years.
However, these are estimated values as the time depends on the factors we mentioned.
Overall, polyurethane turning yellow is common and more common with the oil-based type. The yellowing is due to polymer degradation, UV exposure, or oily deposits.
To fix it, sand the glossy finish or expose the coating (before it has dried) to light. If nothing works, remove the entire coating and re-apply it.
Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,