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Message: <blockquote data-quote="Gonzo 5/1600" data-source="post: 875428" data-attributes="member: 15455">The brown stuff you're seeing on your MIG welds is just slag, meaning impurities that surface in the moulten puddle such as the copper jacket on the wire etc.  Nothing to worry about, it should just chip off.  As far as cleaning before welding, a good sandblasting is best if you have access to a blast cabinet, otherwise degrease the area (I'm sure people have various sugestions) and scuff with a steel bristled brush.</blockquote>