Cause
Copper corrosion is caused when oxygen and water come in contact with untreated copper that is exposed to air and moisture in the atmosphere. This is also referred to as copper carbonate and copper hydroxide.
Remove the contamination with a solution of acetic acid (vinegar); salt can also help to clean a copper surface. There are also many household products available which will be successful.
If untreated copper is required to be painted then primers for non ferrous metals are available. For domestic heating and plumbing systems clean and abrade the copper surface followed by priming and finishing with suitable oil based paint systems.
If the bare copper appearance is preferred then treating with a clear shellac or clear lacquer can be considered.
Note: Black spots can appear when oil based coatings are applied to an unclean surface.