Composite materials consists of at least two ingredients, of which one is epoxy or polyester, and the other typically is fibre-glass, carbon fibre or teflon.
Epoxy consists of two chemicals, the glue, and the hardener, that must be mixed before it can be used. The duration before it hardens, the work-life, varies, but typically is in the range of 15 mins. It typically takes 24 hours before it is entirely dry though.
Epoxy is very hard when dry. It is very difficult to squash, yet quite easy to snap when bent. Fibre-glass and carbon fibres on the other hand are extremely stong when in traction (pulling), but breaks easily when in torsion (pushing). By combining the epoxy with the fibreglass, we get a material that is strong both in traction and in tortion. The composite material is made by soaking the carbon or glass-fibres in the epoxy, to make a kind of epoxy-fibreglass sheet.
Fibreglass and carbon fibre comes in strands, mats or as woven fabric. Do not buy the mats. They are meant for boat builders. For aircraft use, they do not provide the necessary strength.
Since the fibreglass is bendable before the epoxy is applied, we can make very hard shells in every possible shape and size, by first laying up the fibreglass fabric and then applying the epoxy.
We typically use carbon fibre strands to sandwich between thin wood plates, to give it strength. Woven fibres looks like a piece of cloth and is used to make shells like the airplane cowling, and other parts. Woven fibreglass is also used to make a hard shell on the surface of our models. Typically to give strength, and typically where the original airplane that we are modeling had eluminium plates.
Fibreglass fabric fabric comes in several thicknesses and are ordered in weight per area.