Porosity is one of the main weld defects. There is nothing real specific in GTAW (TIG) in regards to porostiy control, but I will outline below the general reasons and way to stop porosity in welding.
Make sure all surfaces to be joined are clean, free of oil, pains etc (this includes filler material) Remove any oxide layers, especially in aluminium. Your work area is shielded from the wind Choose the correct shielding gas. TIG generally 100% Argon is most suited to all appicaltions and metals Gas flow is correct 8-10 L/min (GTAW). Going higher can work against you
Hope this helps you out. Need me to help you with anything more specific drop me a line.