I think the active ingredient is cuprous oxide not pure copper you have to oxidize it somehow to get that red power. 4 Cu + O2 → 2 Cu2O
I am not sure how they make it commercially but I know it is not copper flakes.
Pick the heaviest can... that usually works the best.
Avoid the e-paint, it wears off quickly on the waterline and does not work that well below the surface.
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