A spray tube assembly connected to a barrel of a spray gun used in a rotary spray gun, comprising: a spray tube, said spray tube having an orifice located on one end thereof and a connection portion located on an opposite end thereof, said connection portion defining therein a circular mounting hole for connection to the barrel of said spray gun; a nozzle tube mounted in said spray tube, said nozzle tube having an elongated tubular body portion and a nozzle tip extended from a front end of said elongated tubular body portion an opposite end thereof terminating in at an angle and suspending inside said spray tube in proximity to said orifice: a reinforcing tube mounted inside said spray tube around said elongated tubular body portion of said nozzle tube; a first bearing attached to one end of said reinforcing tube around one end of said elongated tubular body portion of said nozzle tube near said nozzle tip to support rotation of said nozzle tube relative to said spray tube; a second bearing attached to an opposite end of said reinforcing tube around an opposite end of said elongated tubular body portion of said nozzle tube near said connection portion to support rotation of said nozzle tube relative to said spray tube; and a gasket ring attached to one side of said second bearing opposite to said reinforcing tube and kept in contact with the inside wall of said spray tube near said connection portion. a spray tube, said spray tube having an orifice located on one end thereof and a connection portion located on an opposite end thereof, said connection portion defining therein a circular mounting hole for connection to the barrel of said spray gun; a nozzle tube mounted in said spray tube, said nozzle tube having an elongated tubular body portion and a nozzle tip extended from a front end of said elongated tubular body portion an opposite end thereof terminating in at an angle and suspending inside said spray tube in proximity to said orifice: a reinforcing tube mounted inside said spray tube around said elongated tubular body portion of said nozzle tube; a first bearing attached to one end of said reinforcing tube around one end of said elongated tubular body portion of said nozzle tube near said nozzle tip to support rotation of said nozzle tube relative to said spray tube; a second bearing attached to an opposite end of said reinforcing tube around an opposite end of said elongated tubular body portion of said nozzle tube near said connection portion to support rotation of said nozzle tube relative to said spray tube; and a gasket ring attached to one side of said second bearing opposite to said reinforcing tube and kept in contact with the inside wall of said spray tube near said connection portion. a reinforcing tube mounted inside said spray tube around said elongated tubular body portion of said nozzle tube; a first bearing attached to one end of said reinforcing tube around one end of said elongated tubular body portion of said nozzle tube near said nozzle tip to support rotation of said nozzle tube relative to said spray tube; a second bearing attached to an opposite end of said reinforcing tube around an opposite end of said elongated tubular body portion of said nozzle tube near said connection portion to support rotation of said nozzle tube relative to said spray tube; and a gasket ring attached to one side of said second bearing opposite to said reinforcing tube and kept in contact with the inside wall of said spray tube near said connection portion.