My thought was to coat the birch with epoxy and put the trim on. Then in a month or two, I will take it off and install the aluminum skin under it. Does that change the answer or should I seal the spar faces?
I understand better now...
If you are gluing the Birch to the spars, then no, you don't need to epoxy them.
Since the Birch is permanent, I would be sure that the glue oozes out of the sidewall to roof joint, and wipe smooth before it dries.
Then after it is dry, install your trim with roll putty.
Done so, the only place to be concerned with moisture intrusion is the screws/holes holding the trim.
I always install the screws, then remove a few at a time, inject sealer, such as OSI clear gutter seal, and reinstall the screws.
It is removable, not like silicone, or adhesive.
After I installed the aluminum skin, and retrimmed, I'd use the seal along the edges of the trim as well.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...