Automatic solvent injection for plural component spray gun

An air purge plural component spray gun having a mix chamber and a purge air passage selectively connecting pressurized air to said mix chamber, the improvement comprising:

• a source of solvent;
• and
• a solvent piston having a front and a rear and movable between a first non-spraying position and a second spraying position such that in said first position, high pressure air is applied to said rear of said solvent piston to close a solvent bore connected to said solvent source to prevent passage of solvent into said mix chamber and in said second position, high pressure air is applied to said front of said solvent piston on detriggering said gun causing said solvent piston to retract uncovering said solvent bore, closing off a solvent inlet and allowing a slug of solvent to mix with purge air in said purge air passage and pass from a solvent outlet through said purge air passage and into said mix chamber.