Sounds like you know your way around epoxy, but have you tried a bit of heat to encourage the reaction? That will sometimes help.
What brand are you using? Lately I've been using WEST System. I am seeing mixed information about shelf life, but I had not thought it was overly sensitive to that. [Okay, last edit! This is from a source I trust:]
Shelf life is indefinite, unless the materials show visible signs of deterioration. Even partially crystallized product can often be salvaged, though unless you allow it to freeze this is unlikely.
The viscosity may change over a span of years, and the colors may change--particularly the hardener--but in general it lasts a very long time. When in doubt, and when beginning to use an old container that hasn't been touched in some time, it pays to mix a small test batch and allow it to cure to ensure that it works properly.
Of course all of the above might not apply if you using something a bit more newfangled, like G-flex in a dispenser tube set.
Also, are you being really nit-picky about your mix ratios? Are you going by weight
or volume? I'm picturing you using small amounts so it would be critical. On small batches I make sure to add the resin to the hardener cup so that no hardener is left on the sides like it might be if the hardener cup was scraped into the resin cup (since it's a smaller amount, typically).
For regular resin, if it were me, I would be suspecting my mixing ratio of having an error. (see below for some troubleshooting info from WEST).
This project sounds really nifty! I can't wait to see how you go about it, and the results
PS: Here is a troubleshooting guide from WEST System's site on the subject of not curing. For their product, they don't list "too old" as a possible cause, specifically:
The epoxy mixture has not cured after the recommended cure time has passed.
POSSIBLE CAUSES & SOLUTIONS
Off ratio: Too much or too little hardener will affect the cure time and thoroughness of the cure.
1. Remove epoxy. Do not apply additional material over non-curing epoxy. See epoxy removal note in User Manual.
2. Check correct number of pump strokes: Use equal strokes of resin and hardener. DO NOT add extra hardener for faster cure!
3. Check for correct pump (5:1 or 3:1 ratio) and pump group size (Group B resin and Group B hardener).
4. Check pump ratio (see pump instructions). See Dispensing in the User Manual.
Low temperature: epoxy mixtures cure slower at low temperatures.
1. Allow extra curing time in cool weather.
2. Apply heat to maintain the chemical reaction and sped the cure.
3. Use a faster hardener, designed to cure at lower temperatures.
4. See Understanding Cure Time in the User Manual.
1. Remove epoxy. Do not apply additional material over non-curing epoxy. See epoxy removal note.
2. Mix resin and hardener together thoroughly to avoid resin-rich and hardener-rich areas.
3. Add fillers or additives after resin and hardener have been thoroughly mixed
See Mixing in the User Manual.
1. Remove epoxy. Do not apply additional material over non-curing epoxy. See epoxy removal note in the user manual.
2. Check for proper resin and hardener. Resin will not cure properly with other brands of hardener or with polyester catalysts.