So you need a product that is lighter than steel and won't corrode but it needs to be harder than plastic and not as specialized as carbon fiber. If this is the case, then you're in the market for fiberglass. Fiberglass has long been a solution in applications that need the stability and paintability of steel but without steel's tendency to rust. It is easily molded into complex designs that would have to be welded, stamped or hand worked in steel applications. Plus, like steel, fiberglass can be sanded and painted to a mirror shine or have a colored gel coat applied in the mold, which takes away the need for secondary finishing.
There are a couple different ways that fiberglass parts can be made. The final piece and what its specifications are determines what process will be used. The first process used is called open faced contact molding. In this process a one sided mold has fiberglass either sprayed into it or has layers of fiberglass fabric laid down inside the mold then an epoxy applied. Open molding creates parts that require a finished surface on one side only.
The second process is called closed molding, which is done by sucking the fiberglass resin into a mold, creating parts with a finished surface on all sides.
The finishing process of a fiberglass part can also be done in a couple different ways. First off a part can be sanded and painted to a mirror shine or a gel coat can be applied in the mold. A gel coat is a form of finishing that, depending on the quality of the mold, has the ability to create a show quality finish in almost any color. Items such as boat hulls are a common item that is gel coated. The process basically eliminates the need for secondary finishing processes after a part comes out of the mold by finishing the inside surfaces of the mold. When a part comes out of the mold the gel coat will have the same level of finish as the mold.