The spray-up process is similar to hand lay-up except that a mechanical spray system is used to apply both chopped glass strands and polyester resin. The spray system, often called a chopper gun, is connected to resin, catalyst, and glass strands. Air pressure is used to spray both chopped glass into a catalyzed resin stream. The steam is directed toward the mold surface. The ratio and amount of glass and resin is controlled by the air pressure and mechanisms of the gun. The thickness of resin and glass is controlled by the speed of the gun as it moves across the surface. Compared to hand lay-up, the spray-up process requires more skill. It is, however, much faster, and can be automated more easily. Spray up systems are often used to back up thermo formed acrylic plastic sheet for bathtubs, shower stalls, hot tubs, camper tops, cycle fairing, and pool accessories.