The extra cure time also gives the glue more time to "soak" deeper into the wood before it solidifies. Otherwise, the glue might only bond to the outer layer of wood and simply peel the outer layer off. I'm sure 20 minute epoxy would be fine, but I would NOT use 5min. Some people use only 5 min and have no trouble, but I would rather error on the side of safety and use 30minute (plus, I am slow

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Regards,
Philip