Okay, now I have to stick my oar in.
I regularly stick weld 1.6mm wall material, which is often rusted and will probably stick my chassis too.
I have no welding qualification or "coding" except what I do in my day to day work. If I don't like the weld I just did I grind the bugger out and do it again. Grinder discs are lasting longer and longer now.
When I started using stick on this paper thin material I quickly became very proficient at the patching of holes as I found it hard to not blow a hole.
Currently I use about 90Amps (May be slightly less) through a 2mm rod to join 1.6mm, seems to work a charm.
Heaven forbid I lose concentration and blow a hole in my chassis, I will cut the relevant section out and redo it.