Tin Oxide
Several years ago, the EPA pulled tin oxide based paints off the market for general purpose boat, bottoms, but I think it is still used in the special purpose paints for aluminum products (outdrives). Tin Oxide is extremely effective / toxic. It may be the active ingrediant in the transducer paint also. I'm going to try a barrier such as epoxy to isolate the metal surfaces electrically from the paint, then cover with the specialized paint.