'Nother thing worth mentioning is where the culprit moisture is originating. If your safe is sitting on a concrete slab-on-grade (SOG) or your home's sub-floor crawl space doesn't have a 6-mil vapor barrier over the soil and your safe is in an unheated closet, you'll likely have rust and mold.

If it is the SOG, you could try setting your safe on something to keep the moisture vapor from directly entering the bolt holes in the bottom of your safe. Painted plywood, sheet rubber, torch down roofing, etc. set over a bead of caulk around any holddown bolts would probably help matters.