Many thanks for the replies everyone and yes, I am trying to bond this through-hull to a fiberglass boat.
Someone on our blog suggested I try Boat Life LifeSeal (partly silicone based) as they had successfully used it on the same Vetus through-hull. Fortunately, we have some friends on our way to visit us in Panama
and they can bring some down for us. I'm a bit hesitant to use 5200 since I haven't had luck with 4200 and my understanding is 5200 is just a much stronger polyurethane
sealant. So hopefully something a bit different will work.
Thanks again for the help!
belizesailor - when I can stop working on projects, I'll come on over and say hi!