“You don’t think the spider cracks could be in the filler do you?”
To be honest, I don’t know. I have never really seen a wood canvas canoe that has been shellacked up close, but then I don’t remember ever seeing filler have spider cracks.
There is lots of discussion about Shellac on the Wooden Canoe site and maybe Fitz might chime in on your thread, he has some experience with it. In one thread he mentioned making your own shellac vs. using what could be old product on store shelves to prevent cracks, and he suggested light coats vs. a thick coat.