Jay,

My local epoxy expert at work has told me its best to boil the resin to "recondition" it. The resin turns all white and crystalline due to absorbing moisture from the air. When you just heat it up the moisture is till in the resin and will effect the chemistry to some degree. Boiling for 10 minutes will evaporate the moisture completely. He's reconditioned lots of epoxy this way and has done so for me. You don't want the boil to be too vigorous either as you will start boiling off VOC's in the epoxy.

Don't do this in the same microwave/oven you heat up your lunch in!

Matthew