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A spray gun is disclosed for use in admixing and spraying, as a homogeneous mixture, two or more chemically reactive fluid components, more especially catalyzed polymerizable resin coating materials such as polyurethanes, epoxides, polyesters and other thermosets, either of the self-foaming or film-forming type. Because of the high rate of reactivity of the fluid components involved, the gun is designed to restrict actual admixture of them to a relatively small region within the gun body located immediately upstream of the spray discharge orifice so that application to the surface occurs as promptly as practical after admixture. Flow rate control and check valve means are provided in the gun for the several fluid components; in addition a solvent flush system is also incorporated for purging the gun after use. The flow rate control valves are of special ball design, affording greater mechanical simplification and better sealing maintenance than conventional poppet or needle valves. The check valve means are likewise designed to facilitate gun maintenance by the provision of a unitized cartridge, design, making them easily removable from and replaceable in the gun body and giving easier and more complete access to the interior of the gun in connection with periodic major maintenance servicing of it.