Composite can be a misleading term. Wood is a natural composite of cellulose and lignin , adobe is a composite of straw and mud, MDF is wood fibers wax and resins. None of those are particularly water resistant. But when most people say composite they mean something made with a man made resin that most folks would consider to be a plastic or epoxy and designed and engineered to be resistant to water, weather, sun etc. So when buying "composites" make sure everybody is on the same page. Ahh, yes, paper is a composite.