If you're going to have other pieces ready for epoxy in the next day or so, you can stick the unused portion in the freezer. It wont kick off until it warms back up. I've kept half a cup usable for three days in the reefer. When I was ready to use it, I stuck the cup in hot water to get it up to temp and make it not as thick.
Just a thought.
-David

: Mix your epoxy in smaller batches; I use a marked syringe to measure. Try
: this trick for any leftovers: As soon as the hull is stiched together
: build cardboard damns in the bow and stern for the end pours. Then
: whenever you have excess epoxy, thicken it and pour it into the damns.

: George F. Johnson