Moscow Mule copper mugs can get dingy. To keep the outside shiny and looking like new, you need a good polish. Paycok Imports sells Barkeeper's Friend. This is a great and easy-to-use polish that also works on other surfaces. It is important to know that Barkeeper's Friend is a polish and not soap. It is not recommended for use to clean the inside of your Moscow Mule copper mugs. It is for the purpose of keeping the outside of your mugs shiny, only.