What can I use on the exterior of a Featherweight case to clean it up? I have tried a damp wash rag to no avail. Can I use saddle soap? I have a dual colored green and cream case so the black shoe polish trick isn't going to work on that one. It has dirt, grime, smudges, and probably tobacco residue on it (yellow brown color). I searched the board and couldn't find anything relating to this specifically. Ann in TN