In article , Richard Riley wrote:

You know, as I think about it, for your purpose a 2 part expanding
foam is probably better.
http://www.aircraftspruce.com/catalo...es/x30foam.php

Clean the fiberglass thoroughly. Then pour it on the inside of your
fiberglass and brush it around quickly, before it starts foaming
(you'll only have a few seconds) and it will bond well to the
fiberglass.


No, no, no, you dunno wanna do dat! The two part foam will absorb
and trap water (holding it forever). Been derr don dat!