I'd forget using a refrigerator....when you remove the powder from the fridge it will absorb moisture as it reaches room temps from the warm air. Instead, I would get a good tight fitting ice chest and keep the powder in there. Don't use any ice, but at room temps. along with desicant (absorbs moisture and holds it).
I would NOT use anything electric to heat/dehumidify the air inside the ice chest for safety reasons. You can find desicant bags at any boat or marine supply dealership. The other option if you could swing it, is to aircondition your work shop to get the humidity under control, which has a couple advantages and makes the job allot more enjoyable for both you and your equipment (no rusting/corrosion worries).