Get the earth clamp as close as you can to where you are welding, clamp it to the main body frame not to a bolted on wing or other piece that may not have a good earth.

Clean the earth contact as said and if it's a cheap pressed clamp, cut it off and bolt the earth cable to a small "G" clamp or if you are doing a lot of welding drill the body and bolt the earth cable directly to it.

There's nothing worse than trying to weld when the earth clamp has fallen off and you haven't realised.
It's almost as bad as when the weld starts splattering and you think you've hit a bit of rust when it fact you've either forgotten to turn the gas on or it's run out!!

And have some clean tips to hand for when the wire burns back and melts to the tip, saves a lot of time if you clean up all the ruined tips after you've done welding instead of whist you are welding.

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