Helping it to drain
First, polyester resin including gel coat is not 100% waterproof. Epoxy is about 10x more water resistant.As for your rudder... it could be water infiltration through the gel coat, or a crack in the rudder or leakage where the shaft meets the rudder.It would have helped to drill a few holes into the bottom of the rudder to drain it. The faster it drains the fewer deposits it leaves. Then repair the the rudder and epoxy coat it using a barrier coat method.Before that is done, I would make sure that the rudder is solid on the shaft. If there is any play between the rudder and the shaft, I would be concerned as to it's integrity.At that point I would start looking for a new rudder.You have a lot invested in your boat and to jepardize it by repairing a failing rudder may not be the wisest thing to do.Hope this helpedr.w.landau