In my experience, especially with oldies, a lot of the detail on your coppers is in that layer of corrosion.
I've lost a lot of decent coppers through 'cleaning'.

I soak them in several changes of freshwater to leach out the salt, let dry, and spray with a light coat of matte laquer to strengthen.
I like the green patina

Here's an example of a GII that I believe if cleaned would turn out blank.
Looks great as is to me...
Best Digs
