Polyurethane is a durable finish that lasts very long on the material. But over time, the polyurethane 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 works fine. The sandpaper will help to remove the yellow tint and lighten the polyurethane finish.
You should know that in most cases, using ultra-fine sandpaper to remove the yellow tint won’t work. So the best way to fix yellow polyurethane is to strip the finish with an alcohol-based solvent or chemical stripper.
Then bleach the wood using Clorox bleach, apply a stain-blocking primer and apply a non-yellowing finish. But, there’s more to this. We explained everything you need to know down below.
Why Is The Polyurethane Turning Yellow?
Polyurethane turns yellow for different reasons. But, the most common reasons are the oils in the top coat. Also, polyurethane turns yellow due to chemical reactions in the paint coating, polymer degradation, and UV exposure.
Polyurethane turns yellow as the top coat pulls and absorbs oils, tannins, and dye from the material used on. Polyurethane also turns yellow over time due to usage and dust accumulation. Polyurethane turning yellow is more common with oil-based polyurethane.
Polyurethane is a very strong top coat. The strength of the top coat is due to the paint formula. Polyurethane contains many additives including resins and plastics. Over time, a chemical reaction can occur between these compounds leading to discoloration or yellowing of the polyurethane coating.
The yellowish tint is also caused by an over-accumulation of synthetic oils. Over time, the oils in the polyurethane coating can turn the clear coat yellow.
Another reason for yellowing is due to UV exposure. This is very common with indoor polyurethane since indoor polyurethane doesn’t have UV protection. This causes the UV rays to penetrate the polyurethane 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.
Polyurethane can also turn yellow due to the absorbent qualities of the paint thinner or solvent. This is more common with water-based polyurethane. Water-based paints tend to pull or suck materials from the surface. This is why water-based paints are common for pulling it raising wood fibers.
For water-based polyurethane, the solvent can also pull dye and tannins in the wood. If the wood has a lot of oil or stain that wasn’t sealed with a stain-blocking primer, this might be the reason your clear coat is turning yellow or sometimes, orange.
So can you paint over yellowed polyurethane? Let’s find out.
Can You Paint Over Yellowed Polyurethane?
You can paint over yellowed polyurethane. But to do so, you need to sand the polyurethane to remove the top glossy layer first.
Then you’ll have to seal the yellowed polyurethane with a stain-blocking primer like shellac. After, you should apply a non-yellowing finish on the polyurethane to prevent history from repeating itself.
However, it’s better not to paint over yellowed polyurethane. If the polyurethane 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.
No one likes the look of polyurethane once it starts to show that yellowish tint. The good news is that you can paint over it. All you need to do is sand the polyurethane first with medium or fine-grit sandpaper. This helps to abrade the polyurethane so the next coat can sit tightly.
You also need to use a stain-blocking primer like shellac on the polyurethane coating. This is to prevent the yellowish tint or any other substance from bleeding through. Next, you should use a non-yellowing sealant. These sealants often contain silicone compounds that prevent heat or UV rays from turning the coat yellow. A good choice is a rust oleum finish.
For the polyurethane to turn yellow, this means it’s time for it to go. If you take the risk and paint over yellowed polyurethane, the whole finish can come off after a while. Especially, if the yellowing is caused by a bleed through.
Related Read: Can You Apply Polyurethane Over Polyurethane?
How To Fix Yellow Polyurethane (Made Easy)?
There is no quick fix to get rid of yellow polyurethane. This is because when polyurethane 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 of the polyurethane, it won’t work on the undercoat. So technically, the yellow tint will still be present.
However, you can scrub the yellowed polyurethane with mineral spirits and a soft sponge. This can help get rid of most of the yellowing. The mineral spirits will dissolve and remove grease or yellow deposits on the polyurethane.
You can also try scuffing the top layer of polyurethane to remove most of the yellow tint. 220-grit sandpaper will work fine. The sandpaper will help to remove most of the yellow tint on the top layer of the polyurethane.
This hack 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 the yellowing.
But in other cases, the best way to fix yellow polyurethane is to strip the finish with a chemical paint stripper, seal the material with a stain-blocking primer, and apply a fresh coat of non-yellowing sealant.
Related Read: How To Fix Sticky Polyurethane?
Does Water-Based Polyurethane Turn Yellow Over Time?
Water-based polyurethane rarely turns yellow over time. This is because the polyurethane has a very low level of oils in the coating.
Also, water-based polyurethane has a low level of VOCs, paint chemicals, and solvents which means the paint coating is less likely to react or turn yellow.
However, water-based polyurethane can yellow over time based on the absorbent qualities of the paint coating. Water-based polyurethane can pull or absorb oils, tannins, and stains from the wood leading to the polyurethane coating turning yellow.
Asides from that, cheap and low-quality water-based polyurethane products are also yellow over time. However, this takes a while before it happens, usually over 3 years.
How To Prevent Polyurethane From Yellowing?
The following tips will help to prevent your polyurethane from yellowing:
- Ensure the material to be painted is dry
- Apply a stain-blocking primer like shellac before applying the polyurethane
- Use exterior polyurethane because the top coat is designed with UV blockers
- Apply non-yellowing polyurethane like water-based polyurethane
- Use sealants that contain silicone compounds like rust-oleum sealant
- You can also try sealing the polyurethane finish with a coat of UV-resistant sealant.
Which Types Of Polyurethane Yellow Less?
Water-based polyurethane is known to yellow less than oil-based polyurethane. This is because there is a lower level of paint chemicals in the polyurethane.
Also, water-based polyurethane uses water as the paint thinner. So there is no oily deposit or residue that can yellow the paint coating.
Water-based polyurethane is also less prone to weathering effects. UV exposure and heat will not make the water-based polyurethane yellow as it would on oil-based polyurethane. Top non-yellowing water-based polyurethane products include:
- 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 turn white paint to yellow.
As a clear coat, polyurethane is designed to reveal the color of the undercoat or paint underneath. So if you apply polyurethane over white paint, the paint will remain white, only that it will now 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, you should know that it’s not the white paint underneath that turns yellow. Rather, it’s the polyurethane coating that begins to have a yellow tint.
Since the clears coat is turning yellow due to polymer degradation or UV exposure, it can make the white paint underneath appear yellow making you feel that the polyurethane has turned your white paint to yellow.
Related Read: How To Thin Polyurethane Paint?
Overall, polyurethane turning yellow is common and more common with oil-based polyurethane. The yellowing is due to polymer degradation, UV exposure, or oily deposits.
However, you can lighten the yellowing by scuffing the polyurethane finish with a fine-grit sandpaper. This will remove some of the yellowing.
But, is not a guarantee as scuffing is more ideal for removing scratch marks on the polyurethane and not yellowing. The best way to get rid of the yellowing is to strip the polyurethane and start from scratch.