WELDINGER Universal assortment 2.0 / 2.5 / 3.25 mm

By plate thickness and available current, roughly: 2.0 mm (0.08 in) for sheet and light work at 40–70 A, 2.5 mm (0.10 in) for 3–5 mm (0.12 to 0.20 in) steel at 70–100 A, 3.25 mm (0.13 in) for 5 mm (0.20 in) and up at 100–140 A. The packet prints a current range; stay inside it. Too large a rod on thin plate burns through, too small on thick plate leaves a weld with no penetration.

The zinc coating boils off before the steel melts, which makes the arc unstable and produces zinc oxide fume — genuinely harmful, and the cause of « metal fume fever ». Grind the coating back from the weld line where you can, and extract or ventilate properly. No electrode makes that unnecessary.