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Why is there sometimes copper residue after stripping EN?

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I have recently been experimenting using room temperature electroless nickel stripper to take nickel plating off of old nickel plated steel horse bits and spurs. The solution seems to work well and the plating on some pieces comes off clean as a whistle after a few hours in the solution. however some pieces have a copper coating or residue on them which does come off with a little elbow grease, light sanding, and buffing but is difficult to remove from moving parts and crevices ect. My question is: What causes this copper coating? Do I simply need to soak some pieces in the solution longer to get them to come out completely clean down to the steel or is there some other reason that certain pieces come out clean while others have this copper coating? if so how can I avoid this copper residue or what can I do to take the copper off without sanding , buffing ect. Will longer soaking help or is the copper coating just normal with this type of stripping solution?

Matthew Jones
hobbyist - Dallas, Texas

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Strippers are deliberately designed to remove only one metal. Most likely, the parts that came out 'clean' were electroless nickel on steel. The other parts were probably copper and nickel plated before the electroless nickel plating, and your stripper doesn't remove the copper. You would need to follow the nickel stripper with a copper stripper.

Ted Mooney, P.E. finishing.com Brick, New Jersey