A method is provided for preparing a composite product, the method comprising: forming a core structure having a plurality of layers, each layer being build by an additive manufacturing process; wherein the core structure has a shell substantially enclosing an internal chamber of the core structure, wherein the shell is fluid-impermeable, and wherein the outer surface of the shell has a predefined shape; introducing a fluid into the chamber of the core structure; arranging the core structure and reinforcement fibers in a mould, wherein the mould is arranged to accommodate the core structure including the reinforcement fibers, and wherein the reinforcement fibers and the outer surface of the shell are arranged in contact with each other; providing the reinforcement fibers and a resin on the outer surface of the shell of the core structure; and solidifying the resin to form the composite product inside the mould at a moulding pressure Pm, while controlling the fluid pressure PF of the fluid in the core structure.