Pg. 88 – “Thicken epoxy with Cell-o-fill to a mustard consistency and spread it on the mating surface of each stringer.”
Generally speaking, whenever you think of mating two pieces together, think “mustard consistency epoxy”. We used wood-flour in lieu of cell-o-fill. I believe either is acceptable.
(Whenever you see fillet, think “peanut butter consistency epoxy”.)
Mustard consistency epoxy will stand up *much better* to moisture and water than wood glue.
Enjoy!