Seal polished copper
Hi - I'm wondering after I polish the copper - do I need to seal the copper?
It is only necessary to seal the copper after polishing or adding a patina if you wish to have it keep that finish. This includes bright copper also.
If left to its own devices, copper will patina on its own and turn brown, like a penny. It also loses its highly polished appearance. Some folks prefer to let copper be and enjoy the natural patina it develops. I have a few pieces that I did not put a protective coating on. Bright copper highlights come and go depending upon wear, etc. But overall, the pieces have acquired a very lovely finish and patina.
Unprotected copper must be buffed by hand with a jewelry polishing cloth as needed. It can also turn the wearer's skin blackish or green where the copper touches it. Copper is reputed to have some therapeutic qualities, so if one is wearing it for those purposes, you wouldn't want a coating on it.
You can use a number of things to "seal" copper including Renaissance Wax, Permalac lacquer, etc. You'll have to experiment with them to find which works best for your needs.