At the heart of the Spray-Up process is the spray or chopper gun, which simultaneously and uniformly chops the assembled roving, mixes it with the resin, containing initiator, accelerator, filler, pigment and other additives, and directs the mixed stream onto an open mold. The roving is chopped into a specific length. Compaction, wet-out and air-release are accomplished, using a roller as in the Hand Lay-Up process. One good appearance side is possible with this process. The material selection is important. The resin should be easy to atomize. The fiberglass roving should be of moderate stiffness, easy to chop, with good static and ribbon control, ensuring efficient processing.