Welding metal is easy. Welding rust is hard. It's all in the prep - it needs to be shiney bright and thick enough to weld to. If its not then it needs more prep!
Plus you need the right power settings, gas flow, wire flow and of course the biggest thing a decent earth.
I doubt I have any of that with the machine I have. It's a SIP beginers welder, 2 settings with auto speed control.
In the manual it says to slow down the speed, tighten the wire grip which puts more stress on the motor and slows it down !!!!!!