Originally Posted by poopdeckpappy
This is not entirely correct, it doesn't have any load bearing value but it's not for cosmetic purposes; CSM will give you thickness and stiffness but it's the cloth ( E,S or biaxial ) that will give it strength and is the last layer ( excluding the gelcoat ) in the finish product, that is why they use 4 or 6oz cloth on surfboards, strength and appearence
"Thickness" increases the structural moment of Inertia about the neutral axis, matt requires a relatively resin-rich structure and on a cost ratio basis becomes the most expensive way to 'gain such thickness'. The prime purpose of matting to provide a 'cushion' under gelcoat to prevent 'print-through' of the roving/cloth during the very long term 'post-cure' phase of the polymerization. Stiffness is due to the fiberstress along the outtermost 'skin' of a structure, matting does not have the structural capacity to do this, its a 'cosmetic filler' and not much more.