They did not have modern epoxy glues at that time.
Does anyone have a cost-per-square-foot value on carbon-fibre sheeting, thick enougn to build a Mossie out of?
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Well, no, that's why they are called modern.
But there were two other epoxy-based adhesives in common use during WW2, on both sides of the Atlantic Ocean - Aerolite and Redux -
Aerolite - When World War II broke out, Morris Motors used Aerolite and Aero Research's strip heating process to assemble Airspeed Horsa gliders, as did de Havilland on its Mosquito, as well as on other aircraft and also naval launches and patrol boats.
Redux - The first formulation available was Redux Liquid E/Formvar, comprising a phenolic liquid (Redux Liquid E) and a PVF powder (Formvar), and after its initial non-aviation related application of bonding clutch plates on Churchill and Cromwell tanks, it was used by de Havilland from 1943 to the early 1960s, on, among other aircraft, the Hornet, the Comet and the derived Nimrod, and the Dove, Heron and Trident. It was also used by Vickers on the Viking and by Chance Vought on the F7U Cutlass.
Typically, Redux would be used to affix stiffening stringers and doublers to wing and fuselage panels, the resulting panel being both stronger and lighter than a riveted structure. In the case of the Hornet it was used to join the aluminium lower-wing skin to the wooden upper wing structure, and in the fabrication of the aluminium/wood main wing spar, both forms of composite construction made possible by the advent of Redux.
Cascamite - the lactose-based casein resin adhesive, was not the only one used on the Mosquito.
tac