One method is to take a shop light, the kind with a cage around it that hangs, put a 150 watt light bulb in it...place the light in the case with the lid down and let it sit for 24 hours. The heat from the bulb absorbs the musty odor. Another method is to make a sachet of cheese cloth and charcoal and let it sit in the case for a week. Fabreeze helps in some cases. I have two featherweights and I line the bottom of the cases with dryer sheets, after I treated them with the light method. It helped a lot. Good luck you you. You can also sit the case, open, outside in the sun but you take a chance on cats and birds....