I wasn't going to get involved in this one with poly vs epoxy but
Honestly I would talk with a few more professionals, in the yards and suppliers, and get their advice first ... I dont think this is something anyone would want to do twice !!!
I've not heard about the glass spheres or microballons not being able to be used U/W ....Thats a common fairing material that can be mixed with other agents to make suitable fairing compounds... Cabosil is more "structural" and a bitch to fair. Again with talk of a barrier coat you are good to go.
Mat... I have always always been told that fiberglass mat, which is what I would use as the first layer against the damage area,
Chemically and Physically bonds with the poly based resins. There is a chemical reaction between the mat and poly that creates chemical interlinking and bonding …. I think I would definitely want that in your situation. That first layer is going to hold everything together. The understanding I have is that this is not true with epoxy. Depending on how much you need to build up the suggestion is a layer of mat then structural glass( like the 1808) mat then glass. (You can also buy the glass with the mat already stitched in to the 1808 )
Look into Vinylesters ( higher grade polys and even Iso resin) as they are more water resistant ....plus if your doing a barrier coat your protecting it anyway. We all know that they are compatible with your hull...so there is no guessing
Joe Merton would be a good individual to call ...
www.mertons.com he's great to deal with and in Springfield, Ma. and better yet is a SeaCraft admirer.