Filler grade follows the base metal: ER308L for 304 and 304L, ER316L for 316 and 316L, ER347 for 321 and 347, ER309L for joining stainless to carbon steel, ER2209 for 2205 duplex, ER310 for 310S, ERNiCrMo-3 for 625 and dissimilar joints.
MIG wire comes on 15 kg spools and 250 kg drums in 0.8, 1.0, 1.2 and 1.6 mm; TIG rods in 1 m lengths in 1.6 to 4 mm; flux cored in 1.2 and 1.6 mm. Diameter follows plate thickness and welding position.
Look for the AWS classification, the EN ISO designation, batch number and certificate. Poor wire shows as feeding problems, porosity and inconsistent bead — cheap wire is rarely cheap.