Make sure you have a vented PSS. A buddy of mine runs a prop shop and has seen numerous cases of shaft log crevice corrosion due to oxygen starved water. The vented PSS will allow more oxygenated water in the shaft log and can minimize any risk of crevice corrosion. This is also happening with newer "drip free" packings.

This is a shaft that had been using a "drip-less" shaft packing..