If you just want a dull finish, you can take it off and then to a machine shop or automotive rebuilding establishment and have them sand-blast it. That should give it a matte finish, PDQ. A less pretty but quicker and cheaper aternative might very well be using progressively finer grits of sandpaper and then emery paper until you reach a finish that you like. I am using the latter on a project to bring back a badly rusted and pitted sword blade and I then intend to apply them to the cross and to the pommel, both of which a quite bright and really not in character with the blade.