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Rudder Post Corrosion
There are two kinds of stainless used in marine applications 300 & 400 series. Take a WEAK magnet and put it next to the shaft. If you feel it pull, you have a 400 series which is high in iron and carbon that makes it stronger. But the 300 series is higher in nickle and crome that makes it less corrosive. Chances are you have the 400 which is less expensive. Stagnet water creates an acid which will eat away at the iron. ether material do the fallowing if possible.
Remove the rudder and undercut that area up about an inch by machine or grinding. Then fill that area with epoxy and glass cloth and sand it back to the original size. That should seal that area against acidic errosion.