Originally Posted by motorbiker
A mig is constant voltage. The wire speed is the current.
In MIG welding amps are adjusted by changing wire speed. The higher the wire speed the more amps.
For thinner metal you would want to slow the wire speed down and adjust the voltage.
If the voltage is too low the wire will hit the puddle. If the voltage is too high the arc will be too big and there will be lots of spatter.
All MIG machines are not the same. I have settings for volts, amps and wire feed speed.
For deeper penetration on thick material you'll want to slow the feed and increase output voltage. That way the arc stays concentrated longer in a given area, hence deeper penetration. On thin stuff you'll want to move fast to decrease the chance of burn through.
The copper backing idea is a good one for thin materials. The copper absorbs some of the heat and makes longer beads without stopping possible. I've used copper to back up behind a hole I needed to fill in sheetmetal. The steel filler won't stick to the copper and when the hole is filled just pull it off.