Bill's right about the Compass Marine, Butyl tape. I use it wherever I want something really sealed. Since I haven't had my windshield frame off, I can only guess what the joint is like, but if it is as flat as I suspect (alum angle resting on the fiberglass), you may want to used something like 4000 or 4200 for a combo of adhesion and sealing. 5200 would do the job but it doesn't have much flex, not that you'd need it on that joint. Still, using 5200 would make it hard to get off the next time if you even care about that, but it would stay white, where as the 4000 or 4200 would yellow and would need to be painted. Good luck and post pics for us Manatee guys and girls.
"I'd rather be happy than dignified".