Does Polyurethane go bad? It’s given that nothing lasts forever. Nonetheless, knowing the lifespan of a product helps you make the best of it.

Polyurethane, like other finishes, has an estimated length of time it should be usable. However, this duration can be prolonged or reduced based on various factors-we will discuss below in detail.

Does Polyurethane expire? Please find out the answer, along with tips on the best ways to store and extend its lifespan.

Does Polyurethane go bad?

Yes, Polyurethane does go bad, but after lasting you a while. Usually, an unopened can of Polyurethane is estimated to last around three years, while an opened one should be usable for about a year. Polyurethane can last longer if stored in an airtight container and away from fluctuating temperatures. 

What’s the Shelf Life of Polyurethane?

Both water-based and oil-based Polyurethane have the same estimated lifespan of three years. However, the same external conditions affect the two types of Polyurethane differently. Yet you can subject both of them to the same test to determine whether they’re good or expired. 

Does Polyurethane Go Bad in the Can?

If it stays unopened and kept within the recommended storage conditions, it can stay usable for up to 10 years. However, if it’s opened, it has about 12 months before going bad. Still, when stored under freezing to vaporizing temperatures, it can deteriorate faster. 

How Long Does Polyurethane Last?

Polyurethane can last years longer than the estimated time. But once opened, it should be in good shape for about a year, could be more. When it’s applied properly, Polyurethane can last many years. And oil-based poly has a longer lifetime than water-based poly. 

Interesting read: Top-tier oil-based polyurethane for hardwood floors.

Minwax Polyurethane Shelf Life

Oil-based and water-based polyurethanes are projected to have about three years of shelf life. However, it’s also true that these products can have a way longer or shorter shelf life than estimated.  

Also read: Can you put wax over polyurethane?

How to Store Polyurethane

When storing Polyurethane, you need to keep it in a dark and dry place, preferably in a temperature-controlled environment.

An ideal location would be a basement or some closet; the garage might not present a controlled temperature situation suitable for finishes.

Editor’s recommendation: Satin vs semi-gloss polyurethane finish.

How to Extend the Lifespan of Opened Polyurethane

Once you have opened a can and used a bit of it, what’s left needs to be stored carefully. There’re tried-and-tested practices guaranteed to help preserve leftover finishes: keep them usable for several months up to years.

Air is the main threat to Polyurethane’s lifespan because it triggers its curing process. Air can also introduce bacteria and moisture into the container, promoting microbial growth

That said, your first act is to eliminate any air from the can. This can be done in various ways discussed below:

Trans fill a smaller container

First, you can eliminate air space by transferring what’s left into a smaller container, filling it, and securing the lid or cap tightly. The idea is to minimize the space between the Polyurethane and the lid.

Make use of marbles

Another tip you can try, if you can’t find a smaller container, is putting marbles or small rocks in the opened can. The marbles sink to the bottom of the can and raise the level of the finish in the container: this, in turn, eliminates the air space between the product and the lid.

Displace air with Bloxygen

Still, you can expel air from the can using Bloxygen. This is an aerosol product made of inert argon gas. Open the lid, spray the inert air and close the lid tightly.

Bloxygen will displace the oxygen, blanket the finish, and keep it from thickening, crusting, or going solid.

Store the product in a temperature-controlled environment

Another aspect you can control is the temperature exposed to the product. Extreme temperatures can freeze or vaporize the varnish. Such temperature fluctuations can lead to a change in texture, which inadvertently degrades the finish and renders it useless.

Also, read our article on tinting oil-based polyurethane.

How to Tell if Polyurethane is Bad

If you suspect that your can of Polyurethane has expired, there are telltale signs to look out for: For example; texture, smell, and drying time.

Look out for unusual texture

The first obvious sign of a product gone bad is a change in texture. This can manifest in top layer film, which indicates that the product has begun curing. 

Alternatively, it could be that the product has separated into chunks, or Polyurethane is thick at the bottom. Stir thoroughly and if it blends well, brush it on scrap wood. If it applies evenly, it may not be time to dispose of it yet.

Check for sour smell

The following step is to smell the product. If you can tell apart the regular polyurethane smell from a sour polyurethane smell, then you have your answer. 

However, this method is most practical for regular users of these finishes because polyurethanes tend to give off a strong smell. Keep in mind that exploring various polyurethane odor-removal techniques might be beneficial as well.

Look out for its drying time

This test is simple: apply Polyurethane using a rag to a piece of scrap wood or glass and let it sit for 24 hours. The product is still usable if the finish dries evenly and is hard to the touch after the set period.

However, if the finish is still wet or sticky after a day has passed, dispose of it immediately. Find out the exact dry time for polyurethane.

Can you Use Old Polyurethane?

Yes, you can use old Polyurethane as long as you’re sure it hasn’t gone bad – it isn’t hard or has a foul sour smell.

As a rule of thumb, always test the finish on scrap wood first before working on your entire project. That way, if the test results go south, you know what to do.

If Polyurethane has separated or has a clumpy texture, try giving it a good stir to mix it all. Once it’s fairly mixed, filter it into a clean container. Then try applying it on scrap wood. If it goes on smooth and dries in time, use it.

Related: How to Fix Repair Polyurethane

Polyurethane Hardened in a Can?

Polyurethane hardened in the can is a sign the product has started curing. If it’s hard through and through, it should be disposed of properly. But if it’s merely hardened at the top, but you can salvage what’s at the bottom, go for it.

How Long is Polyurethane Good for After Opening?

The standard duration is 12 months. However, opened Polyurethane can last longer if stored properly. You can extend Polyurethane’s lifespan by keeping it in an airtight container while maintaining favorable temperatures in the storage room.

How to Dispose of Polyurethane that’s Gone Bad

Polyurethane finish can harm the environment, human beings, and animals alike. That is why proper disposal is essential. The disposal can be free or done at a cost depending on your local municipality.

  1. Keeping in mind that expired Polyurethane could mean it’s partially cured or is unusually clumpy, leave it exposed outside, so it cures fully.
  2. You could also brush it on cardboard or newspapers, dry it, and dispose of it in a landfill with the regular garbage.
  3. Another option is mixing it with kitty litter and letting it dry out. After the finish dries and solidifies, it can be disposed of with household trash into a landfill.
  4. Find out the designated centers where you can drop off the hazardous waste and procedures and policies involved in the process. 
  5. Alternatively, you can schedule a hazardous waste pick-up by the waste and recycling company or your local municipality.

All the while, ensure the waste product is properly sealed and kept away from any source of heat.

Related Article: Does Wood Stain Expire?


Is polyurethane hazardous waste?

Yes, Polyurethane is indeed hazardous because it contains harmful volatile organic compounds. When these VOCs get released into the air, inhaling them can cause respiratory diseases, among other allergic reactions. When disposed into a landfill, it decomposes into substances that damage the ecosystem.

Does polycrylic go bad?

Yes, polycrylic can transform into an unusable form, referred to as “gone bad .”Like Polyurethane, its estimated shelf life is three years. But, this lifespan can be shortened or lengthened depending on storage conditions.

How long does spar urethane last?

Spar urethane can last about five years after opening, provided it’s stored in favorable conditions. When applied correctly, and depending on the conditions they are subjected to, Oil-based Spar urethane can last anywhere between 10 to 15 years, while the water-based counterpart can last up to 3 to 5 years.

Read also: What is the difference between polyurethane and spar urethane?

Is Polyurethane supposed to be white?

No, Polyurethane should dry clear. If yours appears white, chances are it hasn’t dried fully; give it time. But if the finish is dry but still appears hazy, it’s either moisture got trapped during the application or the poly wasn’t stirred thoroughly before application.


Now, does Polyurethane go bad? Yes, it does, and often the effects are irreversible. But if you catch it early enough, you can salvage the product.

The primary cause of deterioration in Polyurethane is air: even the slightest exposure can lead to gradual curing.

Remember that you can only prolong opened or unopened polyurethane shelf life under proper storage conditions: airtight containers and temperature-controlled spaces.

Now that you know Polyurethane can expire, you may want to read our article on water-based poly vs oil-based poly to find out which one suits your project.

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