I am just starting out welding. The job is to fillet weld 2"x1" 16 gauge (1.2mm) Rectangular Mild Steel Tubing for fabricating a frame. I have a 250 Amp DC welding machine. Can you suggest Electrode diameter and current? I currently have 8mm electrodes. Would they work fine if I used a low enough amperage setting?
In my humble opinion, 8mm rods are way to big for 16 gauge mild steel. You'd need about 250 Amp for that size rod and you'd melt your stock. I wouldn't use a rod any bigger than 1/8". You'd need 50 to 90 Amps for that size rod. A 6013 welding rod should work fine and it's good for ac and dc welders.
Since you're a beginning welder, practice on a few pieces of scrap stock just to get the feel of a smaller rod.