A method and apparatus for producing a metal core sandwich structure that is lightweight and many times stiffer than regular sheet metal, and which is easily formable into curved structures as well as structures having compound curves. In one embodiment, a formed metal core includes a plurality of cells comprising alternating front and rear projections extending outwardly in front of and behind a median plane, with each projection having a bonding surface area or land configured to be bonded with corresponding external metal sheets on both sides of the formed metal core. A plurality of micro-abrasions or indentations are formed on the bonding lands, allowing stronger bonding joints to be formed between the metal core and the external metal sheets by facilitating improved capillary action by the metal core during the bonding process.