I am rather surprised that any underwater metal is mild steel
, much less critical stuff like a rudder
stock and stern tube.
But if so, then galvanize the $#%@ out of them and don't paint. The galvanizing protects the steel and paint just covers it up. There are paint systems that could work- cold tar epoxy I think, instead of galvanizing, but they have to be perfect and not a single
nick for them to work well. Find a yard that maintains steel boat hulls and ask them what to do.
Galvanizing will last a few years, maybe 2 or three, but just like cruisers who anchor
out full time or with mooring
chain, the galvanizing gets eaten away and has to be redone every few years.