On the exterior, there will be a thin, V-shaped opening between panels 2 and 3.    The rule for fiberglassing is that any surface feature you can fit your fingernail into must be filled first.    We used epoxy with silica in a caulking tube.    (In fact this is the excellent pre-mixed WEST System Six10 epoxy, that comes in a dual-chambered tube with a static mixer nozzle.    No mixing required.    Brilliant and fast, but $20 per tube.)