Prop discoloration

After removing barnacles, sanding, and polishing my prop I noticed that it's got reddish spots scattered around the blades. See attached photos. The prop is bronze. It spends summers in Long Island Sound, coated in Lanocote Anti-Fouling goop, which seems to slow down but not eliminate barnacles. The prop gets scraped by a diver once or twice a summer as well. The prop winters on the hard. Any thoughts about what might be causing this discoloration. Thanks as always.

Update: The folks at IP Home Port suggest that the discoloration is actually de-zincification of the bronze, due to no sacrificial zinc or insufficient zinc on my prop shaft. Sounds right: halfway through last summer I discovered that the zinc that was there on launch was no longer there, so the boat was without a zinc for 3-4 months. A zinc was put on mid-summer, and on haul showed significant erosion. I'll start with a new zinc this year and will also hang a "fish" zinc off the transom at prop level.

Lesson: Watch your zinc!

...and check the integrity of your bonding wires.

Bonding system is good, I check every year before launch.

How is the shaft connected to the bonding system? Transmission coupling? Otherwise, it's isolated, isn't it?

Stern tube is grounded, but the prop shaft is isolated from the stern tube by the rubber lining (the cutless bearing) -- I think?

You should also check your bronze thu hulls also. They were being eaten and can fail. The bonding system wires are failure prone. The grounding wires may look just fine and not be. They wick salt water up the wire, and become poor conductors. I checked my stern tube bonding wire when I replaced a cutlass bearing a couple of years ago and had to cut about 30" off the end before I found good wire to splice to. The same problem can happen with all bonding wires because they are in the bilge. The bonding helps protect the stern tube, When I removed the tube to change the cutlass bearing I noticed corrosion under the rubber where the stuffing box attaches. As an aside, I think they should not be made from SS.
When in a marina I use an auxiliary zinc grounded to the engine. I hang it overboard, and it gets consumed, so I know that it is working. I started this after I was in a marina the had a hot boat some where. My shaft zinc lasted about three weeks! I also try to unplugged when leaving the boat because it helps isolate it from bad boats and marina wiring problems.

George
SV Delicia