Here, epoxy is being applied to the other side of the verticals, as well as to the other spar cap. Next, the spar will be placed into the epoxy and clamped just like the previous stage. After curing, you should end up with something like this, lots of squeeze-out everywhere you look. Yep, it's extra weight, but there's little doubt that full contact was achieved. You could either wipe it up or use it for varnish inside the spar.
Polyurethane goes on thinner, and is therefore lighter.
Use it anywhere that ultraviolet light will not be a factor. I sincerely hope that the sun will never shine inside this spar. A pipe cleaner does a dandy job getting to the inside of the breather holes. A four inch I beam was used to ensure straightness in the other, less critical direction. A six incher would be better, but this is all I had laying around.