An apparatus and method for electro-treating one surface of a metal strip alone or both surfaces sequentially. Another embodiment electro-treats both surfaces simultaneously. All embodiments employ a roll having an electrically conducting outer surface around a substantial portion of which is wrapped the strip. An electrically insulating mesh layer is interposed between the strip and the roll's outer surface, and electrolytic liquid floods the openings in the mesh layer and is retained there by that portion of the strip which is wrapped around the roll. The strip is advanced in a downstream direction, and the roll is simultaneously rotated. The strip and the outer surface of the roll are electrically charged with opposite polarities, but neither one is immersed in a bath of electrolytic liquid. Cations flow between the roll and the wrapped-around portion of the strip through the liquid in the openings of the mesh layer.