It is crucial that you clean every part of the instrument that will be covered by your guard. If you don’t clean the metal, a black composite of tarnish, body oil, and dirt will greet you when you remove the guard. Mixing dirt and oil with sulfur in the air results in tarnish; the moist, warm air held close to the metal by the guard when you practice and perform on your instrument creates the perfect environment for these accumulations.
Laquered instruments need only to be cleaned and thoroughly dried before you fasten on the guard. Silver and gold finishes, however, are porous, so they require special care. Use a tarnish remover such as Tarnex, followed by a silver polish like 3-M Tarni-Shield. (Even if you don’t see tarnish on your instrument, it is still there in minute amounts that will wreak havoc on your instrument inside the guard if they are not removed.) Watch what happens when you apply the silver polish. You’ll probably see much more tarnish coming off on the cloth. The polish perfects and “seals” the finish, so that it deflects destabilizing elements such as sulfur.
Repeat this treatment regularly to keep your instrument in top condition.