I used a mixture of techniques to address my chainplates during my mast refit.
The first part, with the mast removed, I cleaned out the holes through the deck and filled the voids with epoxy as shown in this link.
I used a dermel tool to cut and clean up the excess glass and reform the thru deck holes for the chainplates.
I then beveled the outer deck opening around the chainplates. I then placed the chainplates into the holes attaching them to the interior bulkheads. I filled the beveled space around the plates with butyl tape. Pressing it downs and around the chainplates. I then compressed the covers down on the plates screwing the covers down and observing the butyl ooze out. This sealed the opening of the chainplates through the deck. The Bed-it butyl has good adhesion and flexion properties. To date no leaks. Note that there is nothing sealing the insides of the through deck hole. So if there ever is a leak it will be obvious. Because the epoxy filler filled the space of the core, there is less chance for water to get into the core.
Originally published December 2003 *Moved & Edited September 2017 Bedding With Bed-It Tape 5/20/2022 IMPORTANT UPDATE BED-IT TAPE IS BACK PURCHASE BED-IT TAPE Countersunk/Beveled Hole Over the years,
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Stopping future leaks would be just a clean out and refill of the butyl plug.