The present invention provides method of adjusting a thickness profile of a top metal layer of a workpiece using a processing solution. The seal comprises at least one first membrane-like, annular sealing element with a sealing lip which can be positioned against the shaft in a sealing manner; a metallic, annular supporting element, onto which the sealing element is pulled; and a centring surface formed at the outer circumference of the edge area. It is determined whether the cells continue in a constant pattern.