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He wasn't too clear about that. He removed 5/16" strips from each side of the 2x4 to get nice flat edges for gluing up side to side. A 2x4 from the lumberyard has somewhat rounded corners on each edge so they need to be removed to get a flat glued up panel.
Nice work. Here's a tip on the epoxy...I do a lot of fiberglass work so I have shopped around for my epoxy. I buy mine from US Composites. Their 1 gallon table top epoxy kit is $39.50 versus the $150 kit at the big box stores.Also epoxy will degrade in sunlight unless it is protected with a UV topcoat. So don't make a table or counter like this and use it outdoors. Topcoat it with a UV protective finish such as a Marine Varnish, or an automotive urethane clear coat.