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Not all cardboard is created equal.
I use cardboard for rapid and low cost "prototyping". Once the shape and configuration are ironed out, a "one off" using the cardboard, polyester resin with monomer thinner and some fiberglass can result in an effective and durable part.
However, the paper the cardboard is made of comes in a variety of wood pulp, clay fillers and wax additives for processing and gloss. Cardboard that has little clay and no wax makes the best core for polyester layup as it readily absorbs the thinned resin. A quick and ready test is to wet the cardboard. If it becomes soggy and limp in a minute or two then you have the low clay, low wax paper that is desirable.
There was a fellow up Jack McCornacks way who built aircraft using cardboard as the base material. I bought one of his books and it is buried in my library somewhere. But, the main thing I remember well is the choice of cardboard.
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