Welding Consumables

[m_c]

6013 will work fine in mild steel, and avoid the hassles of using 7018.
3.2mm rods will work with 4mm provided you've got a good butt, without too big a gap. You may find that 2.5mm rods will be a bit easier. The good thing about arc welding is you can still weld thick metal with smaller rods, it's just that you go through them far quicker.

7018 does produce better and stronger welds, but they can be a pain to use. You've got to make sure they're kept dry (all rods need kept dry, but 6013 is more tolerant of not being dry), they can be a bit tempremental to start the arc, and they need the end chipped/filed off if you have to re-start the rod due to the way the flux flows.

I keep my 7018 rods stood next to a radiator in the house, whereas the 6013 get left outside. Even if they are damp, they'll still spark, but if I know I'll be using them, I'll stick a handful on top of a radiator.