By George L. Petrie Core material, with closely spaced bleeder holes (not visible) drilled through, is primed with resin and stuck into the putty before the vacuum bag (rolled up, above workmen) is applied. Balsa has been in use for many years, particularly for interior decks and bulkheads. Different types of foam are made of different materials yet share some similarities. The infusion of cored sandwich panels is dicey; test panels of a planned laminate, core material, and resin system verify both the rate and uniformity of the process. Honeycomb coring looks exactly like its name suggests.