Cored fiberglass is great when it works well because it makes the the structure significantly stronger without adding much weight. The only problem with cores is if there is going to be any problem with the structure, it will be in the core!
Cores are like a sponge, soaking in the resin that is covering them. If the core is over-saturated with resin, it will result in a brittle structure. If the core is under-saturated with resin, it will be dry and not as strong as expected.
This one piece of cored sole in our head during the head refit suffered from a dry core. It was just a little soft no matter how many layers of fiberglass I applied over it (I applied 9 layers of fiberglass!) so the decision was made to put more epoxy into the core.
I had two choices, I could either strip off the fiberglass and expose the dry core, or I could make my own access to it. I chose the easier way!
I drilled a pattern of holes in the top of the core, but not all the way through. This would allow resin to soak into the core and saturate it further. The holes were evenly spaced about 1/2 inch apart to allow the resin to soak through and through, saturating everything.
The wonderful part was after the resin had cured, I could test it by stepping on it and if the core was still soft, I could always simply pour more in through the holes. Luckily, this one attempt fixed the whole issue and it became stiff as a board!