If the grime is just ordinary dirt/dust, beat the whites of 2 eggs and with a piece of soft flannel cloth, rub the beaten eggs into the leather. This is a good cleaning method that doesn't use chemicals. If there are oil stains on the case, touch the stain with Sal Ammoniac. Let stand for 5 minutes, then rinse with clear water. Don't rub too hard or you may remove the color of the leather and the finish. If the stain is still there after one application, repeat the process.