Hiya im a carpet and uph cleaner, been asked by a flooring retailer that recommends me to clean a pergola floor for customer,. In entrance to room and thorough fare evidence of dark/black marking from customers black rubber slippers . He is infirm and shuffels with a walker. Manufacyurers have inspected and advised no fault and have suggested ascetone to remove problem.. Any experience or suggestions for this problem? Thanks Simon