A spray gun for spraying a coating composition having a first component and a second component, said spray gun comprising: a spray gun body comprising a carrier inlet, a first inlet connected to a first connection path; a tubular nozzle casing having a nozzle, said tubular nozzle casing being housed within said spray gun body; a hollow spray needle having a longitudinal channel therein, a channel opening at one end of said hollow spray needle, and a second inlet at the other end of said hollow spray needle distal to said channel opening, wherein said hollow spray needle is partially positioned and is slidable in said tubular nozzle casing between a spray position and a closed position; an inlet valve for controlling said second inlet; and a detachable coupling assembly affixed externally to said spray gun body distal to said nozzle; wherein said second inlet, said inlet valve and said detachable coupling assembly are configured so that said second inlet is open at said spray position and is closed at said closed position; wherein: said first component and said second component are maintained separated in said spray gun; said first inlet is connected to said first connection path that is further connected to a spray passage defined by said tubular nozzle casing and said hollow spray needle for conveying said first component to said nozzle; and said second inlet is connected to said longitudinal channel for conveying said second component to said nozzle when said hollow spray needle is at said spray position, wherein: said detachable coupling assembly comprises a frame, a second connection coupling, a valve coupling, and one or more fasteners; characterized in that: the first inlet is configured to receive said first component by gravity; the second inlet is configured to receive said second component by gravity; the inlet valve comprises a pin valve seated within the second inlet perpendicular to the hollow spray needle and configured to be moved perpendicular to the longitudinal axis of the hollow spray needle by the valve coupling to open the second inlet as the hollow spray needle slides from the closed position to the spray position; and to close the second inlet as the hollow spray needle slides from the spray position to the closed position. a spray gun body comprising a carrier inlet, a first inlet connected to a first connection path; a tubular nozzle casing having a nozzle, said tubular nozzle casing being housed within said spray gun body; a hollow spray needle having a longitudinal channel therein, a channel opening at one end of said hollow spray needle, and a second inlet at the other end of said hollow spray needle distal to said channel opening, wherein said hollow spray needle is partially positioned and is slidable in said tubular nozzle casing between a spray position and a closed position; an inlet valve for controlling said second inlet; and a detachable coupling assembly affixed externally to said spray gun body distal to said nozzle; wherein said second inlet, said inlet valve and said detachable coupling assembly are configured so that said second inlet is open at said spray position and is closed at said closed position; said first component and said second component are maintained separated in said spray gun; said first inlet is connected to said first connection path that is further connected to a spray passage defined by said tubular nozzle casing and said hollow spray needle for conveying said first component to said nozzle; and said second inlet is connected to said longitudinal channel for conveying said second component to said nozzle when said hollow spray needle is at said spray position, wherein: said detachable coupling assembly comprises a frame, a second connection coupling, a valve coupling, and one or more fasteners; characterized in that: the first inlet is configured to receive said first component by gravity; the first inlet is configured to receive said first component by gravity; the second inlet is configured to receive said second component by gravity; the inlet valve comprises a pin valve seated within the second inlet perpendicular to the hollow spray needle and configured to be moved perpendicular to the longitudinal axis of the hollow spray needle by the valve coupling to open the second inlet as the hollow spray needle slides from the closed position to the spray position; and to close the second inlet as the hollow spray needle slides from the spray position to the closed position.