Often, part of the epoxy to be used can become its own primer by diluting it with an appropriate solvent and using this diluted portion with brush, roller or spraygun to prime the surface involved. Others, like fiberglass or other fibers, are used for strength. Sand and aggregate reduce the cost drastically. Casting into molds, cavities, onto patterns, cores, to reproduce, fill, cover, embed, protect, is what epoxies do best. Therefore they are generally preferred in demanding electrical, industrial and scientific applications.
Unfilled resins are desirable for desirable for transparency or lowest obtainable viscosity.
Filled resins are best for minimum shrinkage, lowest exotherm, maximum heat transfer, opacity, machinability, conductivity, and many properties depending on the fillers they contain. Normally, laminates are more expensive and more complicate than castings. But hollow laminates for larger tools, patterns and structures are much less expensive, as well as stronger, lighter and more accurate than castings. They need high heat to accelerate the cure to hours or minutes. They are successful where their limited shelf life is not objectionable.