Thru hull tip sealant
 

Will plain ol silicone work as a sealant behind the trim rings on exhaust tips? Will see alot of under water exposure. Or is it better to go with 4200?

I think most will say use the 4200. I have used urethane caulk. I know silicone is good and would probably work but not the best in my opinion. Make sure your transom holes have been sealed (epoxy or fiberglass) real good if haven't already so no wood is exposed also.

Paradigm is right on. Silicone is not for underwater use, and it is usually stated on the product labeling. I do not use silicone for anything on a boat. Fast cure 5200 or 4200 are so far superior. The advice about sealing the raw wood is the most important piece. Even with the urethanes, raw wood edges will come back to haunt you. Always seal them with epoxy, not polyester. Polyester resin has very low adhesive qualities and should never be used to seal wood if epoxy is available, especially raw edges.

Silicone will work ok. As long as you didn't over cut the holes, you'll have a good 3/8" gasket. I wouldn't use it below the waterline though. The problem with some of the better sealants is that they will be almost impossible to remove. As for clean up, 3M make an adhesive remover that takes the dried silicone caulk right off.

4200, simple, or 5200 if you never want to take it off again

USE 4200 I DID AND IT WORKS