You can try injecting a gas into it, I shoot argon from my welding tank into my paint hardener just before capping it for the day, and it keeps it from going bad.
The last ppg hardener I used last summer, was a 6 year old quart and it was good and not a solid chunk of useless solids like if no inert gas was added.
They sell a small shot can @ good paint suppliers. (I think it is nitrogen but would have to check)
I think the trick is to keep air/oxygen out of it.
You can try propane as it is heavier than air and it would displace the air/oxy that enters the can, and you have propane torch handy.
worth a try--
Disclaimer:
I have not tried it on cpvc see-ment
