I know this is a really old thread, but I'd like to follow through (people rarely do and it is always interesting to me how things worked out).
So - the time referenced in my message above (3 years ago) I used zinc prop paint without a primer. It was off the prop completely in less than 2 months.
I did the same thing next year but cleaned the prop and used the primer first, also applied the same treatment to the bronze rudder gudgeons. The result - it adhered in some places (so much that I had to later power sand it off) while completely dissolved in others, in nice round swiss cheese pattern - again in just a few short months.
I.e. - I doubt it protects against much osmosis, if anything it either promotes it or at least simply gets removed very quickly. It also does not help in any perceptible way against barnacles, only making situation worse because now I have to remove both barnacles and that stuff. I ain't doing that anymore.