I usually have an opened, partially used 300 ml cartridge of polyurethane (PU) sealant around that I slowly use for various repairs. It's the type of cartridges used in a sealant gun.
Now PU cures by reacting with humidity in the air and till now, I could not find a way to store them for longer than 3-4 months without the content starting to harden and going to waste. Ideally I'd like to store them for up to 3 years. That would be longer than the shelf life of even an unopened cartridge. (And if I'd have to remove the sealant from the cartridge for storage, that would also be ok.)
Failure modes: Often the hardening starts in the upper part and slowly progresses downwards. So when poking through the upper part with a nail one might still find soft sealant to use. But sometimes all of the sealant will become more viscous at the same time. It would then not harden completely (yet) but cannot be used with a manual sealant gun anymore. Unopened cartridges seem to harden from humidity entering around the end cap – the piece pushed along once starting to use the sealant gun.
What I tried: I replaced the tip of the cartridge with a screwed on, tight plastic cap for storage. It helps somewhat but is not a reliable or long-term solution, also not even when covering the thread in sealant before. It seems that everything that can transport even small amounts of humidity (and hardened PU sealant does) is sufficient to allow hardening over time. For this reason, methods used for silicone cartridges (woodscrew in the nozzle etc.) don't work for PU sealant: hardened silicone protects the silicone below, but this is not true for PU sealant.