A method for making a composite panel, wherein the composite panel has a honeycomb core sandwiched between a pair of facing sheets, includes the steps of (a) coating the interior walls of the cells with a unexpanded thermo-expandable material; (b) bonding a pair of facing sheets to the opposite sides of the honeycomb core; and (c) heating the honeycomb core while the honeycomb core is sandwiched between the pair of facing sheets with sufficient heat to cause the thermo-expandable material to expand and to substantially fill the cells.