A method and apparatus for producing a 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 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 sheets on both sides of the formed core. A plurality of micro-abrasions or indentations are formed on the bonding lands, allowing stronger bonding joints to be formed between the core and the external sheets by facilitating improved capillary action by the core during the bonding process.