+1 on the flap discs.
Clean the metal till it is shiny and has NO rust on it. Looking at the underside you can see the oxidation that was on those sheets.
I have the same welder (Clarke 160TM) and the power settings are easy:
Lowest: 1, 2, 3 (switch on low) 1, 2, 3 (Switch on high) Highest...