You have three different choices (silicones,polysulfides,or polyurethanes).
I would not use 3M 4200 for this application. It will turn yellow within a few weeks. It has poor UV stability. 3M has a product UV 4000 which has good UV stability. However, it is difficult to tool. I used it on my rub rail and to seal around my radar mount.
I want to reseal the joints between my fiberglass arch and hull. I'm still up in the air about which product to use. I'm leaning towards either a boat life polysulfide or a 3M silicone.
You'll hear comments from people about how bad silicones are and that they'll never use them again. Trust me, I've dealt with every type of sealant on a professional level for over 35yrs. and silicones are a quality product. You do need to insure that the substrates are clean and wax free. Plus, you need to make sure that the silicone is approved for fiberglass. I would not go to a big box store and get an off the shelf silicone. If you do you're asking for trouble.
