After replacing the chainplates, we have not been able to get the deck totally watertight.
We've been applying liberal volumes of various goos: mostly Boat Life Caulk.
Yes, it discolors a little. It bonds to almost everything (excepy polyethelyne). It makes a durable, watertight seal. It's not wonderfully flexible, however.
A skilled technician can apply a neat bead. A finger dipped in soapy water can even out the surface.
Some of these applications were not watertight in heavy rain. The suggestion from some is that this caulk gets hard enough that the normal working of the rigging will break open the bond in places.
What's better? Some suggest butyl tape.
We bought a 40' roll of tape from SailRite, and we're trying it out on the starboard side.
This is tenacious stuff. It's "tape" in it's structure. But this stuff was not like working with duct tape or masking tape. This was like someone squirted a bead of putty onto a strip of backing paper and rolled it into a spiral. It seems to stick to both pieces of backing paper equally well, and it's a crapshoot how it will peel off.
I think that the intent is to unwind several feet of the stuff, cut to length and then put long strips around windows, or on long joints and seams. You can mush it down with the backing paper and then — after mushing — peel off the paper. Ideally, it will stick better to the stuff you mushed it into than it will the paper.
It's commonly used to bed portlights into the hull. There's a big flange that you cover with butyl tape. As you tigthen the bolts, the butyl oozes out. I've seen pictures of people bedding their lifeline stanchions by using big sections of butyl tape on the base of the stanchion.
I've seen pictures of folks drilling the deck holes with a little countersink. The base of the stanchion has a proper countersink machined into out. But they deck holes are given a countersink so that the butyl tape will ooze down around the screw.
Since I only need little 6" strips, unwinding a long strip doesn't seem quite right to me.
I've been scraping 6" sections off the backing paper and then mashing it down around the chainplate.
I can shape it into a pleasant little smooth-sided hill between deck and chainplate. I've been using a plastic scraper to push it down into the tiny cracks around the base of the chainplate hoping to fill any voids.
We'll see how the starboard side fares over the next few weeks. My clothes are stored in the aft starboard lockers, so if my boat clothes stay dry, my fix worked.
If my boat clothes can more mildewed than the already are, then we'll be searching for other solutions.