The Anest Iwata Needle Packing Set 93940890 is designed specifically for the mini LPH series sprayguns and RG3 & G series airbrushs. This part sits at the front of the spraygun/airbrush giving an fluid tight seal at the front where the needle enters the front of the gun behind the fluid tip, if you have fluid travelling back on the needle towards the trigger, there's a good chance this is the problem.
