You might look at Incralac and/or Syncralac, the first is thinner based the second is water based (I like the thinner based myself). It was made specifically for copper alloys, goes on easily and if needed (at least for the thinner based material) it can rather easily be removed with MEK then reapplied. Was developed, as far as I know, for use on bronze sculptures - the copper part - and was of removal/reapplication so outdoor pieces could be rather easily re finished, without damaging the patina that was being protected. Both are UV inhibited, the thinner used produce tends to be good outside for a bout 15 years before it starts to craze/get foggy.
Also for all here, you do know that square/sharp edges do not coat well on the sharp edge so getting all sharp corners off of a piece before the protection coat goes on, will give far better, longer lasting protection than leaving the sharp edges. The coating is VERY THIN on the “point” of the sharp edge and that is where most coating fail.
Also a VERY TOUGH clear coat is a POR15.comhttp://POR15.com catalyzed clear coat (http://www.por15.com/2K-Urethane_p_123.html) that is very tough, especially when it gets moist. Drawbacks are it’s cost ($76.00 a qt) and it is a rather thick, finished coating but the thickness might be reduced with thinners and careful application. Give them a call if you would like some input about thinning. Used on yachts for keeping bright work fittings polished yet usable. Toughest coating I have ever had to take off (and THAT is another story,.).
Just a bit of potentially good info…