Knotted, smooth wire. Knotted cup brushes are used in heavy-duty brushing applications, e.g. for preparing metal surfaces prior to painting, for removing slag, rust, scale or concrete residues. The 80 mm and 100-mm diameter brushes feature a support ring designed to stabilise the brush. When the free wire facing wears down, the ring can be removed and a further 20 mm of usable wire length created.