We are working on a project in Glasgow, which uses large random cut blue slate as a cladding material on the elevations. The slate is smooth cut and polished and the colour,tone is constant , however in about 10% of the cladding, a football sized “oil” type stain appears which is supposedly a natural product of the slate itself. It is unsightly and we want it removed by the supplier, who refuse, can anyone advise. Your help would be appreciated