2 pack clear - Hardener went hard!

Hello.
As the sun deigned to come out recently to warm my old bones and announce the start of painting season I dug out my Zero paints 2 pack clear to attempt to get the "dipped in treacle look" on a couple of bodies.
I found that the opened bottle of hardener that I had stored upside down to prevent is forming a skin had hardened around the neck of the bottle rendering it inaccessible without smashing the bottle.
In retrospect a skin is better as I can poke through it with a screwdriver to release the hardener underneath.

Do any of you lot out there know why this happens?  I screw the cap on tightly but that doesn't seem good enough.  The fact that when left standing upside-down it hardened from the bottom up (leaving liquid sitting on top of it) would suggest that it is due to something getting in through the cap.
Air or moisture?
Should I keep it indoors where the humidity is lower.
Maybe in the fridge with my BMF?

dunno about this Zen but i did come across something similar with resin and found out u can buy something called an airblanket, i havent seen or used it but im told that after using the resin, spray some airblanket in the top of the container before sealing it again this stuff somehow displaces any air in the container and prevents the resin going off on its own over time.