Restoring 20' fiberglass boat - Challenge needs solution

Over the years i have built whole decks including side deck , fore deck ,motor well etc etc using glass panels made from 450 csm on one side with 3 mm thick corematt and another 450gram csm on the otherside . Cut to shape with a diamond blade as if you were using plywood , polyester bond together sand and glass over the joins . Can use a hot melt glue gun in places . Point being what you are using is not wasted as it becomes apart of the deck ,for bigger areas make the core thicker and it support its self over a longer distance . if you use core matt its possible to cut it to almost the size and shape you need and just glass it on a flat smooth panel so theres even less waste just a trim sand all over and fit together . Id be looking at vinylester resins instead of epoxy !,it easyer to work with and quicker to make panels from ,once you head down the epoxy path thats where you have to stay !!
Remember this is only to form a shape to build the rest of the layup over to get to the final thinkness with layers of matt and csm . I always use peel ply on the final layer to give a good smooth surface so you can see the final shape for fairing and filling etc
Its even possible to make big panels to do repairs the same way , its only limited to you ability as to the size of what you want to do !.

 

I think that your process is nearly there. Here at Explorer Marine, we build custom fast RIBs and often need to modify an existing moulding - see www.explorermarine.co.uk.
I would suggest that you make some thin flat sheets and attach horizontally to the bottom of the deck. Then apply closed cell polyurethane foam - available quite cheaply now as foil faced insulation - but remember to pull off the foil. Shape this to the desired shapes, cutting or adding as the shape develops. then scarf the deck edge and apply polyester resin and glass mat / fabric as applicable to the surfaces and smooth and gel coat as a final layer.
Yes it takes a bit of time, but it is all quite straightforward with basic workshop tools and you can easily improve the shape during the build process as the form develops.
Good Boating