From all of my research, there is no full, long-term sealant for metal. If it is worn, it will eventually wear off. Silver will tarnish, copper will oxidize, and beautiful color will fade.
For me, I work with a lot of copper and I choose not to seal it. To me, copper is one of those metals that only gets prettier with time. Of course, that 's just my preference.
Of course, there are anti-tarnish tabs or strips that you can provide for your customers to store with their jewelry. These are available from multiple retailers and are a nice thing to include with packaging. Anti-tarnish products protect silver, nickel, copper, bronze, base, tin and gold! To use, simply place a strip or tab near polished metal in an enclosed area, such as jewelry cases, storage bags or boxes. Anti-tarnish strips & tabs have approximate 2 year shelf life after it's put into service.
Do remember, when using most of the above protectants, safety rules apply. Well-ventilated areas, masks and gloves are needed.