I've never sealed mine. I keep in on my bench (lower level, cool, no direct sunlight) and have not had any problems.

One thing though, I've heard about this but I thought I was ask. Does heating the epoxy in a microwave affect it's characteristics? When getting low (and impatient) I will throw them in for 8 - 10 seconds. When it comes out it is quicker flowing but does it affect the strength?