You have just come across an old can of polyurethane paint from a previous job and you want to use it. So you are asking yourself, does polyurethane paint go bad? Here is the answer to that.
Polyurethane paint does go bad or expire but not in the way other household items do. Polyurethane paint like most other finishes goes bad based on the storage conditions of the paint.
Does this mean you can use an old can of polyurethane paint? Yes, but you need to revive the paint first. So how do you do that? How do you even know if the polyurethane is still in good condition?
There is more to know about the shelf life of polyurethane paint and this post digs right into the topic.
Let’s take a closer look.
How Long Does Polyurethane Paint Last?
Polyurethane lasts for an average of 6 years but this is only valid for unopened polyurethane cans. Opened or leftover polyurethane paint lasts for about 3 years.
This is because polyurethane is very prone to air especially if the paint container wasn’t properly covered. Even if the lid was tightly closed, the polyurethane paint wouldn’t last more than 4 years at best – because the little amount of air that got in while closing the lid is usually enough to harden the leftover polyurethane within that period. To fully understand how long poly lasts, you need to know how it dries.
Polyurethane paint doesn’t just dry by evaporation alone. The polyurethane has to harden over time. This means even after the solvent has been evaporated, the finish will continue to harden over a couple of hours. This hardening period is known as curing and it is made possible by oxidization. This means the gradual combination with oxygen.
So if you open a can of polyurethane paint, the solvent doesn’t even need to evaporate because the little air that goes in the can carries enough oxygen to cause the polyurethane to begin to harden. If left over time, the poly will completely cure (harden) in the can.
When this happens, the poly has gone bad. This is why poly doesn’t last more than 3 years, 4 years at best after being opened. But if the can is brand new or unopened, then it can last over 6 years.
But since poly doesn’t go bad based on an expiration date, you can’t tell if your leftover poly is still good judging based on its age alone. So how do you know if poly paint has gone bad?
How Do You Know If Polyurethane Paint Has Gone Bad?
To know if Poly has gone bad, you need to first inspect the paint can for a few signs. If you see any of these signs, the paint has gone bad:
If The Can Feels Heavier Than It Should
The first test before opening the can is the weight test. Just by looking at an object, you already have a feeling about how much it should weigh.
If you lift your polyurethane paint container and it feels heavier than it should. Then chances are the paint has cured inside the can. When this happens, the paint would be heavy because it has hardened.
If The Poly Smells Foul
If the polyurethane paint smells rancid or foul – like rotten meat, then it has gone bad and you should trash it. Don’t try painting with that because you will have a very rough surface.
Poly in good condition should have a very strong chemical scent. When you open the paint, your face should be blasted with a strong smell of active chemicals. If the paint smells nothing like chemicals, then it has gone bad.
If The Container Is Puffed
If the poly can is swollen, then the paint has gone bad. You should not even open the polyurethane if the container is puffed because toxic fumes and odor will be released into the air.
Paint containers become puffed when microorganisms are eating the paint inside the container. These microorganisms eat away at the paint and release gas after. It’s the gas being released constantly that causes the poly container to become swollen.
If Mold Grows In The Container
Another indicator that polyurethane is bad is when mold grows inside the container of the poly. Mold growth inside the paint is made possible by moisture.
This means that tiny droplets of water that have been getting into the paint can probably through a leak. The water inside the can causes mold, usually black to grow in the paint can. The mold growth causes the polyurethane to go bad.
Related Read: Can You Tint Polyurethane Paint?
If The Poly Has Hardened
If you open the poly and see a rock of polyurethane in the can, then it has hardened. Try to poke the poly in the can. If you can feel plastic or stone-like material, then your poly has gone bad. This means the poly has fully cured in the can.
If the poly is good, it would be in liquid form.
But sometimes, your poly might not show any of these signs and still be bad. Also, your poly might show one or more of these signs and still be good. The best way to know if the finish is still good is to:
Conduct A Test
You should test the poly on a surface – but not the surface you want to paint. You should use a transparent material – like plastic to test the paint. Gently apply some of the polyurethane after mixing on the plastic and see how it dries.
Water-based polyurethane will need about 6 hours to fully cure and oil-based can take over 12 hours. When the poly has cured, examine the finish it gives. If it’s smooth, glossy, and hard without any chunks, then it’s still in good condition.
If you notice a rough, unsettled, and lump-filled finish, then the paint has gone bad.
Sometimes, your paint will start to go bad just a few weeks after you stored it. If this happens, it means you have not stored the polyurethane well. So how should you store poly?
Related Read: Does Latex Paint Go Bad?
Can You Use Old Polyurethane Paint?
You can use old poly paint as long as the paint is still in good condition. You will also need to revive the old poly paint before you can use it.
Old poly paint can last up to 3 years if stored correctly. So if you have a can of old poly, you just have to ensure it is in good condition and you can use it. This means you should test it as mentioned earlier to see the type of finish it gives.
A smooth finish means it’s good. A lumpy finish means it’s bad. But even if the poly is good, you will still need to revive the paint. Old poly in good condition is still old poly. You need to revive the finish to make it smooth and usable. So how do you revive your old poly? Let’s find out.
Related Read: Primer Paint Life Span?
How To Revive Old Polyurethane Paint?
To revive old poly paint, you need to add a thinning medium or solvent to the poly. A thinning medium is a gel-like substance that gives the poly a more fluid consistency.
Before starting to revive old paint you should cover your skin, eyes, and face before you open the poly can. You should also cover the workspace because you will be mixing the paint later and it can splatter.
Now, here are the steps you need to follow:
1. Open The Paint Container
Carefully lift the lid to open the container. Since it’s old paint, the lid can be stuck. In most cases, it will be stuck. Gently, use the flat side of a screwdriver to lift the cover at different sides till the lid pops up.
2. Add The Thinning Medium
For water-based polyurethane paint, the thinning medium is usually water but you should ask the storekeeper at the paint store for recommendations. They will ask you what type of poly you have and then recommended the best thinning medium to use.
Gently, add the thinning medium into the paint container. Start slow and small.
The next step is to stir the poly. You need to stir the poly continuously for about 15 minutes until the mixture is fluid and smooth. The longer you stir the poly, the easier it becomes to stir. When you have a fluid consistency that flows well, then the polyurethane is ready to use.
For best results, you can use a paint mixer to mix the paint. A paint mixer is a drill-like device that can be inserted into the paint can yo mix the content. If the poly is difficult to stir, you should use a paint mixer. You can also increase the speed of the mixer to make the polyurethane smoother.
4. Test The Paint
After stirring, test the polyurethane by applying it on a transparent surface. If the poly doesn’t show any lump or impurities after being applied, then it has been revived.
If there are still lumps in the paint, you should continue stirring. You can also add more paint thinner or solvent.
Can Unopened Polyurethane Paint Go Bad/Expire?
Unopened polyurethane paint would go bad eventually. The poly can’t stay fresh and usable forever. At best, the poly will begin to lose its viability after 6 years.
Poly that remains unopened will have a long shelf life. But even under perfect conditions, the poly will eventually go bad. This is because of changes in different factors like room temperature.
But you usually wouldn’t need to worry about this unless you are buying a can of poly paint just to leave it in storage for 6 years.
Related Read: Is Polyurethane Coating Flammable?
How to Prevent Polyurethane From Going Bad?
The leftover poly finish should be stored:
Keep it In An Airtight Container
Polyurethanecures through oxidization. This means air is the biggest enemy to the shelf life of your poly. The poly should be stored in a container that prevents air from getting in.
To store the poly, experts advise storing the paint in its original container. You should also seal the container with plastic wrap or nylon. Then cover with the lid and place the poly container upside down. This would prevent any air from getting in.
Keep it In A Cool And Dry Room
The poly should be stored away from any water or heat. If water mixes with the poly, it can cause mold growth in the can. Heat can also offset the chemicals in the poly causing them to react.
This means you shouldn’t store the paint in the kitchen, laundry room, or bathroom.
Related Read: Can You use Polyurethane On Concrete Floor?
Keep it Away From Direct Sunlight
Protect the polyurethane from direct sunlight because the UV rays of the sun will destroy the poly. The poly should also be stored at room temperature.
Storing the poly properly will extend its shelf life.
If the poly finish is unopened, then you should follow the manufacturer’s instructions on how to store it.
Overall, polyurethane finish does expire but if you store the paint well, leftover poly can last over 3 years.
If you have an old can of polyurethane finish, ensure to test it before you use it – so you don’t ruin the surface.