That's the problem with 5 minute epoxy.
It IS rubbery and almost never fully cures, unlike other types.
I do the opposite with fillets, using the longest cure time...30 min pot life epoxy.
It dries extremely smooth and hard. Soaks into both the wood and cardboard.
Impact would destroy the rocket before the joint.
Probably why I NEVER have "Busted Fins".

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !