Re: Chainplate advice needed
It seems pretty obvious that water has gotten in behind your chain plate at some point. The discoloration points to rust, or worse perhaps water leaching out of whatever is behind the knee wall your chain plate is mounted to.
Looks like you have an access port to look into the knee wall. What do you find in there? Boat builders often hide pieces of wood in places like this (where you can't see them). If there is wood in there it is possible that it has gotten wet and is in some state of decay and may have leaked moisture out the bolt holes behind the plate that has discolored the surfaces. If there is wood inside there it may need replacing.
For cleaning it up: acetone, de-natured alcohol etc. Light sanding with fine grit paper etc.
Re-bed it all with Butyl tape as you seem to be planning to do.
What boat make/model is it?
"The cure for anything is salt water~ sweat, tears, or the sea." ~Isak Denesen
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