That's called PATINA! Some folks actually prefer it.

Anyway, there are lots of clear coat finishes for copper and other metals. I'd get one of the spray finishes. Follow the directions for prep. Some of them promise that they'll last for several years outside, but you'll probably need to reapply every couple of years.

You'll find these products in any home improvement center. I'll attach a link to just ONE of many such products, but you are likely to have quite a selection to choose from in the stores.

Here is a link that might be useful: Click here

Rhizo,
Thank you for your advice. I will get the spray, since I prefer my copper shiny and bright!

Natalie