I would make sure silicone has never been used, silicone won't seal with gel coat. If silicone has been used, sand the area until all silicone is removed, water will not bead up on the surface.
I use butyl tape under the small plate, in the hull cutout sealing the actual chain plate between the hull plate and chain plate. The butyl tape has very good properties for stretching and maintaining a water tight seal. Butyl tape remains pliable as when applied even 20 years later. I have re-bedded all my deck hardware, and stanchions with butyl tape. Using butyl tape is time consuming, can't get it on threads of bolts or screws, so needs to be placed on accurately, but you will only have to do it once for most likely as long as you own your boat.
My boat use to leak at the chain plate and numerous deck hardware locations where silicone, 4200, and who knows what else on my 1976 Islander 28, but now I'm leak free. I owe it all to this guys website and source for buying the right stuff!!
Check out the "Re-bedding Deck Hardware" and "need butyl tape".
Compass Marine "How To" Articles Photo Gallery by Compass
Marine at pbase.com
He is also a member here on the forum, but I can't remember his user name. I did all my thru hulls as he recommends on his site, as well as the replacement of two of my port windows. I also used the "sealing Deck Penetrations" on a problem area replacing a hatch
Always more then, "one way to skin a cat", mine is just one.