I do not know what you try to do, but base on my experience the gaps
supposed to hallow/empty. What is my experience ? I built 38ft
sailboat Spray.
I use corecell as a sandwich construction. The idea was to keep
sandwich as light as possible. After corecell was installed I used
very light putty – to fill gaps only on the top.
Next was sanding and on the top fibreglass with resin.
In short – do not fill the gaps with resin.
I hope it helps.
Mada