My solution...
...to chronic chainplate leaks was driven by extreme frustration. Ed is right...this can be a hard job to do right. After doing all the recommended - and unrecommended - fixes, none of which held up very well (possibly there was some movement between the deck and the chain plate) I did the following: I thoroughly cleaned and sanded the areas under the little covers, and about a half inch more. Then, using that kind of easily molded epoxy that comes in a stick, I fashioned a lip that is a bit more than a quarter of an inch high, mounted on the deck, slightly outside the edges of the covers. The lip formed a dam to contain a very thick gasket of 5200 that I floated the chainplate covers on, at about the top level of the lip. I screwed them down to this level, squeazing out some 5200. The area had been masked, so clean up was no problem. The lip is not structural. In the end, I have a tenacious seal of 5200,about a quarter of an inch thick, surrounded by the epoxy lip. For those who shudder at the thought of removing the 5200, I left space between the lip and the chainplate cover, so that I can run a knife underneath. Actually, in my experience, the main reason you would ever want to remove the chainplate covers is when they leak, so if I can make them REALLY leakproof............you get the picture! By the way, so far, so good. Incidentally, the 5200 was thick enough so that it took over a week to really cure....hence the little lips to hold everything in correctly. They don't look bad, either, and the covers are up, off the deck, which makes them less inclined to leak, in the first place. Good luck, and as has been previously mentioned, your leak may be something else, entirely. Check your stanchions!