Soak them for a day in Simple Green. Swish 'em around and rinse off. Put a cheapo steel Goodwill tray in a plastic bucket do the baking soda and water, battery charger trick for another day. Presto! Clean non rusted nuts, bolts and washers for a couple bucks. There are a lot of ways to do it if you are not in a hurry. If you are, buy a tumbler but, you still wait overnight and grease your walnut shells in the process.