I too think that a structural patch should be made, instead of just a surface fill. Where Trillium
glassed the two halves of the shell together, it was done with just a thin layer. This layer was enough to hold the two halves together, but would allow way more flex that the shell itself, and no cracking on the surface would happen due to the separation in the halves, and the decorative band covered it up.
Actually, the band was not just decorative, as it and the clips riveted together held the to shells in line with each other, and does provide a bit of structure that way.
I chose to grind out the seem, remove the loose clips, add one layer of fiberglass cloth (as a backer for the holes/gaps created by the grinding, ad a few more layers of fiberglass matt, and then use the fibreglass reinforced Bondo filler. I am very confident the joint is now extrememely strong.
If just patching the joint without doing a full joint like I did, it would not do much to stop cracking in the filled joint, as the filler really has little area to bond to, and offer little strength to the joint.
Here is a link to what I did. If I was to do it again, I would do exactly the same thing, with maybe just a bit more care to not go onto the shoulder of the band profile, but that really did not cause me much grief. About 10 hours work total, and I figure just properly replacing the belly band strip would take at least half that time anyway.
1976 Trillium 1300 Remodel - For Caitlin