I've kept uncured epoxy in a freezer for several days without it hardening. I don't know really how long it will keep. As soon as it begins to warm up, it will continue curing. Heat accelerates the cure. Epoxy will slowly cure in low temps above freezing. Try making your walnut dust/flour with a belt or other power sander. Just don't go too fast or it will be scorched and thus too dark. I mix mine to a smooth paste that won't run. Too much dust makes it lumpy. Too little may be too thin to fill your gouge. Experiment with some scrap walnut before you do your gun, including sanding and applying finish. But I've been amazed at how much I've been able to steam out bad dents with broken wood fibers and thus skip or minimize filling. My method discovered by trial and error involves removing the finish around the damage, degreasing, and wetting the dent with rubbing alcohol. I allow this to really soak into the wood. Then when I apply my heating iron over a damp pad, I am generating steam/vaporized alcohol from deep within the wood rather than hoping my damp pad forces steam down into the wood. Usually takes several tries, but worth it if you can skip the filler.
