Polyurethane Shelf Life

Article - December 29, 2008

This article was inspired by a question from Terri:

I make signs out of wood and last Spring I purchased a gallon of Helmsman Spar Urethane since I go through quite a bit of it (and absolutely love it). I had used a little more than a 1/4 of the gallon then stored it for the summer in the basement. Its a living basement therefore, the temperature was generally around 65-70 degrees through out the summer. When I opened it this fall to start using again, it had a hard coat over the top and about 4 inches of gel under that. I don’t know if the can wasn’t closed tightly or what happened. I was able to break through the hard coat and tried to mix it all up as best I could but now I have small ‘chunks’ of the gel that gets on my wood work. Is there anything I can do to eliminate this? Should I stir it with a drill attachment paint stirrer or would that cause more problems? I would really hate to have to throw away 3/4 of a gallon so would appreciate any help you could send my way.

Unfortunately my friend, your Urethane is half way into the trash. A skin and chunks are signs that the finish has already started curing in the can. Varnishes cure via oxidation, which means air is your worst enemy. So either the can wasn’t sealed well enough, or the little bit of air that was trapped in the can was enough to keep the curing process going. The key is to put the varnish in a container that has little to no room for air. Or, you can use some Bloxygen. Its an aerosol can full of inert gas that creates an oxygen-free pillow on top of your finish, protecting it from oxidation. Another option is to fill the can with marbles or some other heavy item so the liquid level is raised to the top. But sad to say, this is all for prevention. Once the finish starts curing, as it has in your case, its best to just throw it away. But, if you are feeling especially frugal, feel free to strain out the chunks and test the remaining finish on scrap. If it cures properly and seems to bind strongly to the wood, you can probably get another use out of it. In most cases though, I don’t take any chances once the finish starts curing in the can.

Discussion

Justin57 December 29, 2008 6:46 am

I like to buy in bulk also (including meats/vegetables from Sam’s Club/Costco/etc.), and break down the big cans of vegetables into quart or pint mason jars and vacuum seal them. If oxygen is the culprit with the urethane, would vacuum sealing them lengthen the lifespan of the unused urethane? If so, would the mason jars be cost effective (with food you can reuse the jars, but can the urethane be cleaned out so the jar can be reused?)?

Reply
- thewoodwhisperer December 29, 2008 9:46 am
  
  Hey Justin. If you can find a good safe way to vacuum seal smaller portions, that would indeed be a great way to lengthen the life of the varnish. The built-in seal on the mason jars would definitely be a good way to keep the finish out. But over time, as you suspect, dried finish could become a problem on the lid. It looks like Howard (below) has a great suggestion for simulating that seal with any jar, and its even more cost effective.
  
  Reply
Howard Orridge December 29, 2008 6:57 am

My favorite prevention is to use kitchen cling film applied to the surface of the finish (thereby excluding any air) and then putting the lid back on. Cheap, effective and not too risky!

Reply
- Arlene November 12, 2017 6:44 pm
  
  brilliant!
  
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Millie June 18, 2014 1:29 pm

When I opened an old and partially used can of Helmsman Spar Urethane, I tried it out on a white background to see if indeed it was still clear, as the label stated. It is a bit yellowed when applied. I assume that is also a sign of curing, and I just need to replace it. Can you confirm?

Reply
- Marc Spagnuolo June 18, 2014 3:59 pm
  
  The material will always look yellow on a white background, regardless of age. These finishes do get more yellow over time, but oil-based always has an amber/yellow tone to it. To test if it’s still good, apply some to a piece of wood and make sure it cures in a reasonable amount of time, like 12 hrs or so.
  
  Reply
  - millie June 19, 2014 4:31 am
    
    Good to know…thank you. I need something that will dry clear, yet be durable. It’s for the final protective coat on top of a white wooden stool. Recommendations?
    
    Reply
    - Marc Spagnuolo June 19, 2014 6:14 am
      
      A water-based polyurethane should do the trick.
Ian September 2, 2015 4:36 am

I buy cases of Mason Jars at Walmart to keep my finishes. The trick is to fill them right to the top to remove as much air as possible. As the finish gets used, I dump some cheap marbles in the jar to keep the same level. Mason Jars come in large and small sizes, are cheap, and easy to label. I also do this for my household paints. When my kids make a mark on the wall, easy to break out the jar and touch it up. I find it keeps paint and finishes for years without any issue.

As a side note, Lee Valley just came out with these new “finishing bags”. Haven’t tried them yet but they look pretty cool.

Reply
Pete Matthews Jr December 8, 2017 8:36 pm

My wife had an old, unfinished cart that she wanted to put a cutting board onto, for use in the kitchen – it’s between the too high counter and too low table in height.

Down the basement, I found an unused can of Minwax satin polyurethane, and a little can of Minwax stain in a suitable color. Stained it yesterday, polyurethaned it this morning… still tacky over 12 hours later, but looks OK to me for a first coat. The polyurethane has to be 20 years old – I’m a slow starter! Can this project be saved?

Any help will be appreciated. Thanks – Pete

Reply
Randll L Ford December 17, 2017 10:16 pm

I just found a can of spar urethane that was sealed hat I would like to use. I opened it and it was still liquid but it was thick on the bottom. I stirred it and it seemed to mix completely. The manufacturing date was January of 1995. I am concerned whether it will cure or not.

Reply
- Marc Spagnuolo December 19, 2017 5:01 am
  
  Apply some to a test board. It’s really the only way to know.
  
  Reply
Bruce GRAHAM July 16, 2018 6:21 pm

On another Site , I looked at, which was aimed at Varnish which was Oil Based, (Traditional Boat stuff !)but not Polyurethane….one Guy said to try , as he had done, pouring a small amount of Linseed Oil on the surface, which would spread , and exclude the air, before closing the can. It worked for him. I live in NZ , and just bought a Pint size can of CABOT’s High Gloss Polyurethane based Varnish….as it was NZ $ 33 (! have a chuckle !), I am determined NOT to use the top Inch or so, and Bin the rest ! I also like the ideas of 1) When finished with the can for while, store the can upsidedown or 2) Put a layer of Gladwrap on the surface before closing the can . I have two x Half Pint cans of “Monocel” High Gloss, full of JELLIED Varnish ! I am also determined to re-jevenate that lot too ! I have tried Spirits, which the Australian Manufacturer’s recommended ,but that didn’t work ! When seyttled weather comes once again, I am planning to tip this ‘Gel” into a larger can, and try raising it to almost boiling point, on a outdoors Gas Ring.Anyone got other ideas, I’d appreciate learing about them ! Incidentally, when I went To Sea in the late 1950’s all Cadets (* Like me!) owned a book called “The Boatswains’ Manual”, in which were very good ‘Recipes’ for making Paint and Varnish…..I remember helping the Boatswain to make Clear Varnish, with Turps , Linseed Oil and something else…I lost the Book , and can’r recall the full Recipe ! Please Publish my E.Mail…..so I can read any replies ….Thanks !

Reply
- Gerald Wescott January 5, 2019 2:54 am
  
  https://www.instructables.com/.....d-Varnish/
  
  Reply
Edwin Dreyer October 18, 2020 9:59 am

I have also suffered huge loss in time and redoing a surface coating after applying old polyurethane. Buying large container seems cheap at the start, but I have now learned to buy smaller one litre containers and have no more tears!

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Steve Cook February 8, 2021 11:13 am

Applied Varathane liquid plastic coating to a wood TV tray that was stained from the manufacturer. The Varathane might be 20-25 years old.
It is still tacky after 72 hours after setting in front of a wall heater.
Have you a solution, keep waiting, or strip and recoat with a new purchase of Varathane?

Reply
- Marc Spagnuolo February 8, 2021 3:53 pm
  
  Tacky after 72 hrs in heat is a bad sign. It could very well cure over time but depends on how much time you have to give. I’d personally strip and try again.
  
  Reply
Bill K March 7, 2021 11:24 am

When you store paint, varnish, polyurethane or other finishes store the can upside down. It prevents air from leaking in the can.
Make sure the top is on securely.

Reply
- Marc Spagnuolo March 7, 2021 5:43 pm
  
  This might be a dumb question but if air can get in, can’t liquids also seep out?
  
  Reply
  - Deb Clarkson March 29, 2021 9:18 am
    
    The air is already in the container when you put the lid on.
    
    Reply
    - Marc Spagnuolo March 31, 2021 12:55 pm
      
      Got that, but that doesn’t answer my question. Bill said to turn the can upside down to prevent air from getting in. What I’m saying is if air can get in, there’s a leak. And if there’s a leak, turning it upside down might result in finish seeping out. So it’s probably just better to seal the can properly to avoid air getting in or liquid coming out.
  - Bob Tatar July 5, 2021 9:26 am
    
    Yes if the opening is large enough liquid will leak out. If you can is properly closed however, any liquid able to seep out the tiny opening will harden and this will form a seal that will prevent further liquid exit and also prevent air from entering.
    
    Reply
    - Craig R Larson October 30, 2023 2:58 am
      
      It is not as simple as storing cans up-side or down-side. Chemical, physical and thermal reactions will cause a vacuum to develop in the can and suck in air. Notice the negative dents in your well-sealed but less-than-full cans? Whether the can is stored upside-down (that is what i do), or not, ambient air will be sucked into the can and start the curing process. I hate waste too, but the product you apply to your prepared work better be qualified to justify the project effort. Don’t let a 15 year-old product wreck the work of what you do today.
Cheryl Cole April 8, 2021 6:44 pm

I am refinishing my sons kitchen cabinets, they are formica and i did all the top cabinets last year with chalk paint and 2 coats of minwax poly. They turned out great. So doing the bottom cabints now, thought i was done and my son carried me and said it doesn’t take much to nick the finish off. Not sure what the problem is, is there something wrong with the poly or is it something i did wrong

Reply
- Steph May 27, 2021 12:38 pm
  
  Formica is a smooth surface, and paint doesn’t stick very well to smooth or glossy surfaces. I know a lot of the chalk paint companies swear their paint is “ready to use”, that you don’t need to sand, but for a smooth surface like Formica I would absolutely try to scuff it up a bit before painting, it gives the paint something to stick to. High traffic areas tend to lose their paint quicker, like the bottom cabinets, they’re probably used the most and more susceptible to getting banged into, so you end up with lots of knicks and chips coming off. Next time sand first, use a primer, sand that, and then paint, lightly sanding with 220 grit sandpaper in between each coat. It’s a pain I know, but the results are worth it. Good luck to you!
  
  Reply
Doc.Joe June 4, 2021 11:55 am

I just want to know if Varathane – Water Based Ultimate Polyurethane Varnish, kept in a sealed ‘clear’ glass jar that’s been sitting in the SUN for 7+ hrs a day over 3 full days, which had a daily High Temperature between 75°F to 85°F,& also it is still liquid and still appears to look exactly the same as it did to begin with…..
Will it still be fine and protect the table??
Or does the Sun somehow degrade the liquid and chemically change it??

IT’S MY FINAL(4TH) COAT on a late Mother’s day present for my mom-a beautiful bedside table w/3 shelves that has taken 2 months to build & get it to this point… So I want to make sure it’s going to be okay to use.

Thanks

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Michael Russo August 16, 2023 2:40 pm

I
Bought an5 gallon bucket of poly, and never opened it. Recently came across it and it felt light than it Should have. Anyone have any idea why? Also what the shelf life would be on an opened can? Thank you

Reply

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Polyurethane Shelf Life

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