Re: Rebedding chainplates
Only sealing the top is correct - for all deck fittings. If you seal top & bottom a top leak will go into the core or elsewhere into the boat.
You should remove the chains to check them for crevice corrosion where they pass through the deck.
When you have the chains out, take the opportunity to seal any exposed core with epoxy. Then chamfer or countersink the top of the hole they go in to create an "O-ring" for the sealant to squeeze into.
I, myself, personally intend to continue being outspoken and opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.