Indoor Rusted Pieces:
No maintenance is usually necessary for pieces mounted indoors.
Outdoor Rusted Pieces:
The finish is a real natural rust that developed over time outdoors. I applied a protective clear to protect it, but over time, the patina will continue to develop.
During the first year a thicker, more permanent later of rust will form. This is a good thing because the rust is a very good protective layer and it will allow these pieces to last a very long time. After the first year the appearance does not usually change much, if at all.
The steel is thick enough to last a lifetime without any upkeep. I personally think they look better with age and like the rustic look.
If you ever do want to preserve a certain look though you can use a little steel wool to polish the copper and can apply another coat of clear.
I recommend Rustoleum Matte Clear. It comes in a spray can, is inexpensive, and is available at your local hardware store.