Glass fiber mats are made for subsequent use in preforming for an RTM or SRIM molding process, for example, such that the binder applied to the mat has two distinct curing stages. In the first stage, a partial cure is provided by visible light upon a free radical generation in response to a photoinitiator mixed in the binder in an exact ratio to give a predictable increase in viscosity to that of a semi-solid so that the fibers are sufficiently bound for subsequent handling, but not sufficient to complete a cure, while leaving a second stage ready for a final cure which is achieved by the use of a cationic-type second photoinitiator which is sensitive to ultraviolet light. During the second curing stage, the mat is formed into a three-dimensional shape of a desired end product and cured with ultraviolet radiation while still in the mold, to obtain a rigid three-dimensional preform structure.