......I discovered that much of the rudder stock was rotten due to fresh water penetration. Fortunately the rudder had very substantial teak cheeks either side of the rudder stock, through riveted. It was obviously those which held the rudder together. In my case water had gotten in where the fiberglass had worn through at the point where the tiller fitted into the stock. So it is all about keeping the fresh water out. And of course if the rudder is then fiberglassed the water is not able to dry out. Almost better not to fiberglass a solid rudder. When I built the new rudder I glued a plywood cap over the end grain of the timber.
I'm glad you brought this up. This makes two of us at least who have had the same experience. We serve as a bad example for our members. By "bad example", I mean ,an example that teaches.
Before I forget, Gougeon recommends fiberglass over Douglas Fir to prevent surface checking.
The tiller was fitted to the rudder was very tight. No Chafe. That leaves the bolt holes as possible entry way. I would use the Gougeon method of drilling oversize, sealing and plugging with thickened epoxy for redrilling to bolt size next time.
About corners. Yes, there is the danger of over sanding. There is also the danger of stress concentration. I can't say often enough, as wonderfu as wood epxy construction is, you must pay close attention to inside and outside corners. They MUST be rounded and carefully covered with glass, if using glass. Fillers can be used after for cosmetic adjustments.
In their article, mentioned elsewhere, Gougeons don't like plywood for cantilevered rudders and centerboards because ply is not as fatigue resistant as solid wood. It's also more vulnerable to getting wet because water can migrate so easily along the grain.
I am no expert on moisture meters but a have a simple one. I drilled multiple little holes through the fiberglass cover of Hobbit's rudder, spaced to receive the pins on my meter. I did this in a grid fashion and marked the moisture % readings at each location. It gives me a partial picture of what's underneath. One could draw a "hydrographical" map of the moisture. LOL.
I will probably drill 3/8" holes through the rudder (plugged later) in a staggered grid and bake it in the sun to extract moisture. If I had a suitable vacuum pump, I'd use that. Don't laugh, this method, using a far more sophisticated procedure, is used to extract un-catalyzed liquids from fiberglass hulls with osmosis problems.