On a holistic level, copper has historically been associated as the body metal, and is considered a grounding metal for the human body and spirit. (Silver is the metal of the spirit, and gold is the metal of the mind.)
Spiritually, copper is also considered a metal capable of storing healing properties holistically, as well as having protective properties. This is why it was used in ancient times for totems and talismans.
Because holistic and spiritual uses of the copper properties are holistic in nature, copper can be coated for these properties to work, in theory. However, some people still believe these properties could be blocked by the physical barrier of a coating on the copper. For healing purposes, the copper should not be coated to allow direct contact with the skin.