However to last this long the portlights have to be sealed to prevent water intrusion UNDER the coating at the margins (and attachment screws) or the corrosion/verdigris will 'travel' under the Urethane, etc.

If you want 'brown' instead of 'green', just occasionally wipe with olive oil, ... starting with 'bright' bronze; and, knock down any 'just appearing' green with lemon juice (citric acid). Thats how its done on outside bronze statuary.