This is a responce I wrote for another message a while ago:
Who ever came up with the silly idea to run the chain plates up through the deck should be keel hauledÖ the long way! But as youíre probably not ready to mount phosphor bronze, Herreshoff style chain plates to the outside of your pristine white fiberglass hull, weíll have to try to correct the problem as it stands.
I faced the same problem on my C22. Over the years, water running down the chain plates had caused the wooden bulkheads to rot out. Although I replaced the bulkheads, I wanted to find a way to keep from having to do it again. I sealed the new bulkheads in multiple coats of epoxy as well as the chain plate bolt holes and bedded the plates and the doublers in epoxy too. But what I really wanted to do was to stop the deck joint leak.
First you need to understand what youíre trying to do. Youíre trying to seal a joint between two pieces that move independently of each other, ie, the chain plate and the deck. The movement between these two is caused by the alternate loading and relaxing of the area and the rigging. Although in most cases itís very slight, it still will eventually break down the bond that any chalking may make on the two, unless there is sufficient space for the chalking to flex properly.
I would suggest that you remove the plates, first marking the position where they exit the deck. Also, if the dirt line is gone, mark the area covered by the cover plates as for esthetic purposes, you donít want the repair to extend beyond this. Mark a cut line Ĺ inch around the surfaces of the chain plate, leaving ample material for the cover plate screws to bite on. Cut this area away, right down through the deck, being careful not to damage the mounting brackets for the plates below. Once thatís done, thoroughly clean and dry the area.
If moisture is present, you can use heat lamps and acetone to dry it out. Then seal the core, deck and inner pan surfaces of the cut out with fiberglass. I would use epoxy instead of resin for this, as the repair area will be small. Donít forget to run some epoxy into the cover plate screw holes with a pipe cleaner.
Once the repairs are cured, clean the inner faces with a file, acetone and a scrunge pad. You want to get these surfaces clean and free of wax so the chalk will bond. Use a sander on the deck so the cover plates lay flat and below to clean up the over-head as well. Clean the chain plates and their mounting surfaces below. Use a wire wheel on the sections that pass through the deck, and re-mount the chain plates and doublers. I bedded mine in epoxy but 5200 may work here just as well.
Now, from below, tape off around the chain plates at the overhead then take your 5200 topside and fill the voids around the plates well. A plumberís acid brush may help make a better mechanical bond to the fiberglass and the metal as a first step, then fill Ďem up!
It takes about 7 days for 5200 to cure. It expands a bit as it sets and here is where youíre going to put the final finish on your fix. Using a sharp chisel cut off the excess about ľ inch below deck level at the chain plate. This will give the 5200 some flexing room before it hits the cover plate and is stripped off. Put the covers back on and youíre done! They should give you years of leak free service. Mine were still dry when I sold Olde Blue four years later.
Well, I hope this response wasnít too exhaustive and is helpful to you. Good luck!