Epoxy as a void filler

I have just brought a Contessa a a project and its my first sail boat. I have been pokeing aroung in the inside of the cabin and it looks like the cabine liner has delaminated i.e it feels spongy to the rear the mast support, in the head/locker compartment and above the V berth. I have drilled some holes in these areas to see if the inner core has rotted -and it looks like there never has been any inner core just a void then the outer fiber glass - as I move back to where the liner is not spongy there is a inner core that looks fine and without delamination. I'm not to sure if these voids should be there nor if I need to do anything with them, although I would prefer to fill them with an epoxy just to make a solid liner/ inner roof. Has anyone any experience with this and what would be a good epoxy to fill the voids?