Since you have now gone this far, I would be cutting the whole side off the rudder, inspecting / repairing the entire internal framework, and then reglassing it back together, finishing with several coats of waterproof / barrier coat of some sort. I would also add a large, wide ring of 5200 where the rudder post goes into the rudder, as there is sliight movement / flex here, and it will eventually let water in. I think a good inch or two of 5200 on the rudder top and post (adhered well t oclean, sanded surfaces) will keep water out for a long time!
I would take all paint off the lower attachment point as well, and inspect / repair it ( both on the rudder and the keel where it attaches)
good luck! You will be able to have confidence in your rudder when you are done.