With TIG the rod is chosen exactly as a MIG/MAG wire is, to suit the parent material. For mild steels you take the equivalent of ER70S-6, at Böhler EMK 6 in the classification W 3Si1. For stainless steel 308L, 316L, 347 or 309L depending on the grade and the type of joint. For duplex 2209, for superduplex 2594. For creep-resistant steels the DMO, DCMS, CM 2 and CM 9 families, and note that the manufacturer is gradually moving from designations such as ER90S-B3Si to ER90S-G, so search by trade name.
The rod diameter should match the material thickness and the current. For 1 mm sheet take a 1.0 or 1.6 mm rod at 30 to 60 A. For 3 mm a 2.0 mm rod at 90 to 130 A. For 6 mm a 2.4 mm rod at 140 to 190 A. Heavier sections are welded in runs with a 3.2 mm rod.
Remember to match the tungsten electrode. For DC most often WC20 or WL15, for AC on aluminium WL20 or pure tungsten WP. Grind the electrode along its axis, at about 30 degrees at low currents and 60 degrees at high ones.
Gas shielding has to cover the root as well. On stainless steel and titanium, without a purge the underside of the weld will oxidise and lose its corrosion resistance. Argon or a nitrogen and hydrogen mixture is used for purging.
Store rods in closed, dry packaging. A greasy or oxidised rod causes porosity in the weld.