Job finished and updated! I edited the first post to include the remainder of the job, here is the update:
This past week I finally finished the repair work and am excited to have it done. Aside from the hull blisters, this is probably the biggest repair job on the boat. The next step was to grind down the remaining core and get a fairly smooth surface to glue in the new core. Here is the surface post grinding:
I purchased some end-grain balsa to use as the replacement core. Here is the first piece measured and cut. The board underneath it is the backing plate I will use to hold the core in place while the epoxy dries. I covered it in wax paper so epoxy wont stick to it.
After measuring and cutting the core, I painted both contact surfaces (core and sole) with unthickened epoxy to penetrate, then slathered on a ton of thickened epoxy to the core:
Next I smushed it up against the sole undersides, put on the backing plate, then used shower curtain rods to snug it up.
I filled in the edges with thickened epoxy and let the first piece cure. Here it is after it dried:
I then performed the same tasks on the second piece of core. Here are both pieces in place, with the edges sanded round:
Next up I applied a few progressively larger layers of biaxial cloth to the undersides. I used smallish (1 ft or so) strips to keep it manageable while laying up the glass overhead.
ob finished! I will sand it down and paint it along with the rest of the engine room ceiling. I stood on the cockpit sole for the first time in a while and it is solid as a rock!
Lessons learned: doing the job from the underside was a mistake. My initial idea was that working from the underside would ensure the top skin is undamaged and finish/fairing work would be minimized. The glass underneath is rough and unfinished, so the repair work need not be super neat.However, I underestimated the effort it took to glue in the core overhead with gravity working against me. It was extremely messy, with epoxy getting all over me, in my hair and everywhere. My arms were noodles after each work session working overhead. If I had to do this again I would definitely go from the top and just take my time with the finishing work, particularly since I am fairing and painting the decks regardless. Gravity working for you is a good thing. All in all, I am happy with the repair and glad to be moving forward!