My research found:
Clear Protective Coatings for Copper and Copper Alloys, Publication TN41, 1991
Recommends a product called INCRALAC which is a product that provides a non-dulling formulated protective coating with the addition of benzotriazole as a corrosion inhibitor.
"The International Copper Association has funded the development of coatings specially formulated for use on copper and its alloys. This work has resulted in a number of recommendations but the only
coating available commercially as a direct result of this work is "Incralac". "Incralac" is a solvent-based, air drying acrylic resin coating which can be applied by any of the methods commonly used for solvent-based coatings. It contains an addition of benzotriazole which acts as a most effective corrosion inhibitor ensuring that the underlying metal will not tarnish even where the film is damaged. It has a high clarity and is resistant to degradation by ultra-violet light."
Talasonline.com had the best prices I could find in 30 minutes of GOOGLING and a huge variety of related products, ie., cleaners, polish, rags, etc.
They carry INCRALAC in several forms (water based, solvent based, aerosol can,):