honeycomb structure 10 the threads may be rigidized by the application thereto of a thread enveloping or permeating coating this coating may be thermosetting or thermoplastic in nature or may otherwise be caused to set so that when hardened it encapsulates the threads in a sheath of rigid material when hard this sheath will have substantial compressive strength which will vary in degree depending upon the characteristics of the coating material selected it will be apparent that where the thread selected is porous or can be permeated by the coating material an even stronger thread to coating bond can be assured the coating material may be of many types suitable thermosetting resins include phenolic polyester epoxy and urethane resin in addition to being thermosetting or thermoplastic the resin may incorporate a catalyst or otherwise may be so constituted as to cure at room temperature thermoplastic resins may be acrylics vinyls polystyrenes and cellulosics and may be appropriately selected to withstand ambient conditions of temperature moisture etc to be expected in the panel 39 s ultimate environment in addition the plastics may be of the polyamide type which rigidize upon exposure to air or they may remain fluid until fixed with other substances such as polyisobutylene and polyethylene or boron fluoride whichever coating material is selected an appropriate rigidizing technique is employed to render it hard and firm so as to impart the necessary rigidity and compressive strength to the threads when.