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Most of the times all you have to do is use hot water and a small amount of mild soap to remove accumulated dirt, dust, sweat, etc. from the surface of the metal. You can use your fingers to thoroughly remove dirt from the entire surface with soapy water.
When you have finished washing the piece, rinse it carefully to remove all the soap.
Dry the metal thoroughly. Brass tends to retain traces of water, so you should dry it immediately and carefully.