> My question (finally) is
> - how much is 'a generous radius'? Do I just want to round them over
> enough to help the fiberglass make the turn? Or, do I want to really
> soften the look of those edges? Should I be getting down into the middle
> and lower plys?

You don't want to get down into the middle and lower plys; they are a different color if nothing else (darker). And you probably don't want to get too thin in the wood, either, even if it is filleted on the inside.

I had pretty good results in my Pygmy Cohos by sanding the panel seams down to ALMOST through the top 1mm layer. The radius can be spread something like a 1/4" on either side, depending on the angle of the panels, e.g., where the panels meet more flatly, little sanding is needed to get a good smooth transition, and you won't need to cut very far into the ply.

I used 100 or 120 grit paper on a regular rubber sanding block. Starting somewhere less visible (the hull or rear deck) sand lightly until you get the hang of it. Just as you start to approach the glue boundry with the lower (middle) layer, it will darken a bit, and you can see this. Stop there!

If you go too far, the revealed 'sliver' of underply will be dark like the thickened epoxy in the joint, but even with a bright finish small mistakes will be invisible from more than a couple of feet away. Honest. If you're painting the hull, they won't show at all.

Where the panels meet at more of an angle, you sand more than if they met more flatly. Sanding the top 1 mm layer will give you a much more smooth/round seam than without sanding, and the cloth will wrap nicely, but it won't be quite like as in a strip built. (Nothing is, >sigh<; you can't make it invisible.)

On the multipanel hulls and decks of the Coho, the final result is a much more 'soft' overall appearance, as opposed to a more angular appearance. Your boat has fewer panels, so I'd expect less of that effect, but I'd still think it worthwhile as the clothwork is a bit easier, i.e., it will lay tighter to the wood surface on either side of the seam without bulging or lifting.

This is all assuming you are using the marine grade plywood, okuome (sp?), with the 3- 1mm layers in a 4 mm sheet. Luan plywood, as at Home Depot et al., has a much thinner outer skin, and so there's much more risk of sanding through as well as less of a final softening result because you have so much less to work with.