Does Polyurethane Go Bad? Here’s What You Should Know!

After a few home improvement projects, you’ll have leftover paints and finishes. Also, you may stumble upon an idle can of polyurethane when looking for materials to complete a task. But it is wise to pause and check if the formula fits a new painting job and answer the question, Does Polyurethane Go Bad?

Yes, polyurethane goes bad, but after the specified duration. Generally, an unopened polyurethane can last about three years, while an opened one is usable for about one year. In addition, the formula can last longer if you store it in an airtight container and away from fluctuating temperature levels.

That said, check out this discussion for more insight into polyurethane’s life span and various tips on how to handle the product for a satisfactory project outcome.

How Do I Know If My Polyurethane Is Bad?

There are a few telltale signs to check for when you suspect that your polyurethane is bad. First, evaluate its consistency and look for a film layer or chunks that show if the formula is curing. 

The other strategy involves smelling the product and assessing whether you sense a sour scent. Or better still, you can apply the formula to a piece of wood or glass and allow it to dry for 24 hours. If the finish dries within that duration, it is okay to use.

Below is an in-depth interrogation of various tests for better application.

  • Texture

Image of Minwax for Mahogany DeckExpired polyurethane has an unusual texture. It manifests as a top layer film, indicating that the finish is setting. Alternatively, the formula could be thicker or separated into chunks at the bottom.

Stir the polyurethane thoroughly and observe if it blends well. Then, brush it on scrap wood. If you get an even finish, it is not time to dispose of the product yet.

  • Smell

The other step involves smelling the formula and looking for a sour smell. However, this method is most practical for regular polyurethane users because the product tends to give off a strong odor.

  • Drying Time

This test is simple as you only need to apply polyurethane to a piece of wood or glass and examine it after 24 hours. The formula is still usable if it dries uniformly and is hard to the touch.

On the other hand, dispose of the polyurethane if the finish is still sticky or wet after a day. However, find out the exact drying time for the product to avoid judging it too soon.

  • The Polyurethane’s Can Weight

This weight test should probably precede all the others as it only involves checking how much the container weighs. So, lift the polyurethane paint container, and if it feels heavier than expected, chances are it has cured.

  • A Puffed Container

The paint is not fit for use if the polyurethane can is swollen. In addition, it is advisable not to open the puffed container. Otherwise, you risk releasing its toxic odor and fumes into the atmosphere.

Paint cans are become puffed when microorganisms begin eating the paint inside the container. As a result, they release gas which swells up the container gradually.

  • Mold In the Container

Another indicator that polyurethane is bad is when mold grows inside the can. Usually, this occurrence happens because of excess moisture, meaning that tiny water droplets have been getting into the container. This water causes black mold to grow in the polyurethane can, causing the formula to go bad.

  • Hardened Polyurethane

Your polyurethane has hardened if you open the can and see a rock in the can. Also, check if you feel a stone-like or plastic substance as you poke the can. This observation shows that the formula is dry and expired.

Remember that the product is good when it is in liquid form. But sometimes, the polyurethane may show some of these signs and still be good. Or it might not feature the above symptoms and be bad.

So, the best way to know if the finish is still okay is by conducting a test. Test the polyurethane on the surface you want to paint and see how it dries. A water-based product will need six hours to cure, and an oil-based one requires 12 hours.

Also, examine the finish you get when the formula cures. It is suitable for use if the surface is glossy, smooth, and hard without chunks. On the other hand, the paint is bad if you get an unsettled, rough, and lump-filled product.

How Long Does Polyurethane Last Once Open?

The standard duration for polyurethane is 12 months after you’ve opened the can. In addition, the formula can last longer if you store it properly, like by keeping it in an airtight container.

It is also advisable to maintain favorable temperatures in the storage area and ensure that the formula does not sit outside in the elements.

How Do You Keep Polyurethane from Drying Out?

Polyurethane comes in various can sizes and is highly durable, making it a popular option for woodworking projects. However, half a polyurethane container may not do much, especially if it sits on a basement shelf. Therefore, it is prudent to learn a few tricks on preventing the leftover formula from drying.

You’ll need a few tools for the project. They include a rubber mallet, paint thinner, clean paint rag, liquid polyurethane, a can opener, and small empty glass jars with lids. Then, follow the tips below.

  • Use a can opener to open the polyurethane container carefully. You can also use another tool, but ensure that it does not distort the can lid or rim.
  • Clean the excess formula on the rim and lid before covering the container. This way, you ensure that no air enters the can and dries the finish.
  • Press the lid on the can and cover it with a clean paint rag to prevent spatters. Then, tape it with a rubber mallet.
  • Position the can upside down on the shelf to trap air or varnish fumes and prevent air seepage.
  • Add some paint thinner into the polyurethane to cover the finish. However, do not stir the thinner. Instead, let it float on top. Also, please note that this strategy works if the can lid is intact or when you are uncomfortable with inverting the product during storage.
  • Store the formula in a cool location as high temperatures increase the vapor pressure, leading to air seepage.
  • Store small polyurethane leftover amounts in resealable glass or baby food jars with well-sealing lids. In addition, match the size of a clean and empty glass jar to the remaining polyurethane after a project. Then, pour the formula into the container and wipe the lid to remove drips and spatters.

NB: Polyurethane cures when it contacts air or when the varnish fumes escape from the can. The more air the formula comes into contact with, the faster it hardens.

Also, extreme increases in pressure and temperature cause the finish to leak from the can when you store it upside down.

Why Is My Polyurethane White In the Can?

Polyurethane appears white if it is not completely dry. But if the surface cures and remains hazy, either you did not stir the polyurethane properly before application, or it has trapped moisture. Hence, give the finish time to see whether it becomes clear over time.

In addition, the chances are high that the polyurethane finish will turn milky when you apply a quick-drying formula to a damp surface. The finish hardens before trapped moisture evaporates.

Can You Put Polyurethane Over Old Polyurethane?

Yes, it is possible to put polyurethane over old polyurethane. However, please use the correct application steps to achieve the desired finish. In addition, consider sanding the surface with a dull 120 to 150 grit sandpaper for a better outcome.

You’ll need a few tools for the project. They include a pair of work gloves, fine-grit sandpaper, bristled or foam paintbrush, painter’s tape, paint thinner, drop sheets, a degreaser, and a can of the polyurethane finish. Then, check out the procedure below.

  • Clean the Old Polyurethane To Remove Dirt and Grime

The first step is to prepare the workstation by removing any furniture or object you do not want to paint. Then, place a drop sheet on the floor and use painter’s tape to hold it in place.

Remember that painting can be pretty messy, requiring you to prepare the workspace to handle the mess. In addition, the surface may have grease, grime, and dust that compromise the finish. Therefore, it is best to clean the old polyurethane to facilitate perfect adhesion.

Clean the old polyurethane using a clean cloth. You can also utilize a degreaser such as TSD or rubbing alcohol to remove tough stains.

  • Sand the Old Polyurethane

Sand the polyurethane surface before painting it with a new formula. This way, you scar the old finish, helping the new coating soak into the wood and improving bonding and paint adhesion.

Rub fine-grit sandpaper against the old polyurethane coating. Start with 150-grit sandpaper and then move gradually to finer-grit sandpaper such as 320-grit.

However, please avoid using sandpaper less than 150 grit on the surface as polyurethane sands easily and quickly. Also, the coarse grit will remove the entire finish.

  • Thin the First Two Polyurethane Coats

The next step is to thin the first two coats of the formula. Usually, you do not need to thin the formula, but the finish may take hours to dry before having a new coat. So, it is prudent to thin the first two layers to facilitate faster drying.

In addition, thinning helps the finish flow uniformly to all the nooks and crannies of the wood. Thus, you’ll get a better result by pouring some polyurethane paint into a bucket and mixing it with the recommended paint thinner.

Oil-based polyurethane requires 100% acetone or mineral spirit for the thinning formula, whereas a water-based finish needs only water to thin the paint. Also, ensure that you thin the solution up to 70% as it fastens the drying time significantly.

Nonetheless, do not thin the whole polyurethane can. Instead, pour enough formula to cover the first two layers into a plastic container. Then, keep the remaining finish until you get to the final coat.

  • Apply the New Polyurethane Over Old Polyurethane

Once you stir the paint thinner and polyurethane inside the bucket, it is time to apply the paint. Use a paintbrush or spray gun to apply the finish, depending on how fast you want to complete the work.

In addition, get a synthetic or natural brush for oil-based polyurethane and a bristled brush for water-based paint formulas. You can also use a foam pad for complete and uniform coverage.

Remember to lightly sand between polyurethane coats to improve adhesion and bonding. However, it is best to give the first finish enough time to cure before adding the next paint coat.

Lastly, apply the final coat with a foam brush or bristled paintbrush. Also, consider using long and straight strokes during paint application. This way, you’ll avoid ugly brush marks on the final coat after drying.

  • Spray Aerosol Polyurethane On the Finish

Spray aerosol polyurethane on the surface when the final coats cure. The formula is fine, durable, and glossy, giving the finish an extra touch of class and durability. In addition, it comes in a spray, and you can apply it directly.

Also, leave the polyurethane surface to cure for a few days and avoid using or placing any object on it. This way, you give the paint enough time to harden without scratches and imperfections.

How Do You Remove Haze from Polyurethane?

Image of polyurethane floor but Can I Remove Polyurethane Without Removing Stain?Polyurethane turns milky if it has trapped moisture in the finish, especially when applying the paint in humid conditions. In addition, you will observe some haze due to zinc oxide build-up.

Zinc-oxide is a flattening agent that manufacturers use in polyurethane to disperse reflected light and cut the gloss from the sheen. However, it collects at the can’s bottom, requiring you to stir the formula thoroughly before use.

Unfortunately, if the finish feels hard to the touch and still shows a cloudy appearance, you have a problem. There is no easy strategy to correct a flattening agent or build-up. Even worse, applying another polyurethane coat will not eliminate the cloudiness.

Regardless of the cause of the cloudiness on your surface, you cannot live with it. You’ll probably scrape, strip, or sand the polyurethane varnish. Also, you can check out the following tricks if the haze is not widespread on the finish.

  • Cover the affected area with petroleum jelly or mayonnaise.
  • Wait overnight and then clean the wood. You may notice a positive change because the oil in the above products leeches the moisture from the paint finish.
  • Repeat the process if the cloudiness is still there.

On top of that, it is advisable to prevent a recurrence of the above situation. A few tips to help you avoid this problem include;

  • Stir the formula until the stir stick is no longer covered with white residue when scraping the can’s bottom.
  • Allow the surface to dry to the touch before applying the polyurethane. Also, test it with a few water drops and check whether they soak into the wood. If so, the lumber is too wet to paint.
  • Avoid building up coats of semi-gloss finish or satin. However, if you must have these coats, do it with a gloss product before using satin for the last coat to cut the sheen.

How Long Do I Need to Let Polyurethane Dry?

Oil-based polyurethane needs from two to 24 hours to dry. However, this duration may lengthen or shorten depending on the type of formula and environmental conditions such as humidity content and temperature levels.

More specifically, water-based polyurethane requires a minimum of two hours to dry to the touch, while an oil-based finish needs up to eight hours to dry properly. Also, it is okay to start using the surface after three days.

Frequently Asked Questions

The frequently asked questions include:

  • Can You Put Water-Based Polyurethane Over Old Oil-Based Polyurethane?

You can put water-based polyurethane over oil-based polyurethane. But it is essential to sand the surface to remove the wax on it.

However, it is not advisable to cover oil-based polyurethane with a water-based finish because, in most cases, the topcoat will not adhere properly to the surface.

Remember that water-based and oil-based polyurethane have different chemical components and characteristics. For instance, oil-based polyurethane dries into a glossy and slick finish, making it difficult to add any paint or finish to it.

Therefore, you need to sand the oil-based polyurethane to remove the wax or glossy sheen for a water-based paint to stick to the wood. Otherwise, the latter will not soak into the surface, leading to peeling and early failure.

  • Can You Put Oil-Based Polyurethane Over Water-Based Polyurethane?

It is okay to put oil-based polyurethane over its water-based counterpart. However, it is best to ensure that the previous finish dries thoroughly. In addition, please sand the water-based surface to improve bonding.

Unlike oil-based polyurethane, water-based formulas do not give a high glossy sheen after drying. Moreover, though shiny, the finish is more textured than an oil-based polyurethane surface. Therefore, you only need some light sanding to deliver an excellent topcoat.

Additionally, it is advisable to wait the recommended duration for the water-based polyurethane to dry. Otherwise, the oil-based polyurethane will not stick properly to the surface.

  • Can You Apply Exterior Polyurethane Over Interior Polyurethane?

Yes, you can apply exterior polyurethane over exterior polyurethane. But there is a caveat. Ensure that the finished item or surface remains indoors. Otherwise, it may collapse due to harsh environmental conditions.

In addition, remember that the interior polyurethane serves as the base coat. So, the whole polyurethane coating will peel off sooner or later if the base is not strong enough for exterior conditions.

Polyurethane has two variations; exterior and interior versions. Both products have similar properties. But the exterior formula has additives making it more resistant to harsh weather and UV rays.

On the other hand, interior polyurethane does not contain additives. Hence, it is not resistant to extreme weather conditions and UV rays. This difference makes exterior products tougher and more durable than interior ones.

Nonetheless, it is still not a good idea to use exterior polyurethane over an interior finish on an item that stays outdoors. The result will not be durable, and you will go back to square one with your project.

You may think that the exterior polyurethane will protect the interior finish underneath from harsh weather and UV rays. But the former is not 100% resistant to moisture and UV rays, and the constant exposure to these elements will wear down the surface.

Also, please note that this gradual wear of the exterior finish will expose the interior base coat. And since the interior paints easily break down with UV rays exposure, the finish will peel off quickly.

Lastly, if you use exterior polyurethane over interior polyurethane for an outdoor project, the finish will only last a year before it begins to peel off.

  • Is Polyurethane Hazardous Waste?

Polyurethane is a hazardous waste because it has harmful volatile organic compounds. These VOCs get into the air and cause respiratory diseases and allergic reactions when people inhale them. Even worse, these components damage the ecosystem if left in a landfill.

  • How Do I Dispose of Polyurethane that’s Gone Bad?

Since polyurethane can harm the environment, animals, and human beings, adhering to the recommended disposal strategies is the best way to go. The disposal can be free or costly, depending on your local state rules.

Some of the general recommendations include;

  • Remember that expired polyurethane could mean it is partially dry or unusually clumpy. Thus, leave the formula outside so it cures fully.
  • Brush the product on newspapers or cardboards, dry it, and leave it in a landfill with other regular garbage.
  • Another option is blending the formula with a kitty liner and drying it. Then, dispose of with household trash after the finish dries and solidifies.
  • Schedule a hazardous waste pick-up by your local municipality or a waste and recycling company.
  • Find out the designated areas where you can drop off the expired product and the policies and procedures that apply.
  • All the while, keep the waste formula properly sealed and away from heat sources.


Generally, nothing lasts forever, not even your ideally stored polyurethane. Hence, check the product label and manufacturer’s recommendations on the product’s lifespan to make the best of the formula. Also, read the above write-up on;

Does Polyurethane Go Bad?

Yes, polyurethane goes bad, and the effects are irreversible in most cases. However, you can salvage the product if you catch it early enough. Additionally, it is possible to prolong the formula’s lifespan by storing it in airtight containers and temperature-controlled spaces.

It is also advisable that you get an almost perfect estimation of the amount of polyurethane you will need for a project before you go for shopping. With the right estimations, you will not have to worry about storing your already open cans of polyurethane.

I hope that this post had helpful information even as you try to understand more about polyurethane. If you have a question or opinion regarding polyurethane kindly drop it in the section below and I’ll get right back to you.

Image of a woodworker wearing hearing protectors for woodworking

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.

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