I have found that Acraglass, green or red box, never gets truly hard like the better epoxies; Devcon 10110, Marine Tex et al.
The late Mel Smart (precursor to Serengeti stocks) showed me how to properly bed using Acraglass. It is more work than using better epoxy. I used to send me carbon fiber chips so he could reinforce his bedding. He wouldn't use it without reinforcement. He theory was that it was like pouring concrete without rebar. He liked to use coarse steel wool in heavy areas too!
When I have thick sections to fill, I still use strands of Kevlar or Carbon fiber- it makes a noticeable difference. I know thousands of bedding jobs have probably been done with strait Acraglass but using that red box stuff without thickening it up is really tricky and doesn't yield as good as results as otherwise.
As far as the epoxy going bad, if you can get it to soften up in the jar (I just leave it in the Arizona sun for a few hours) it will set up just fine. Always leave a little in your mixing cup so you have a coupon to prove what it is doing under your action.
