Smartseal rust remover is effective for dissolving problematic rust stains caused by iron-containing fertilizers, metal garden furniture, leaking heating systems, etc on natural stone and concrete surfaces. It can be used externally on patios, driveways and walls as well as internal surfaces if necessary. Generously apply neat to a dry surface and work in with a brush. A purple discolouration shows the start of the reaction. Allow to work for about 5 min, 10 minutes maximum. Do not allow the product to dry. After working time, rinse off with plenty of water. Important: working time must not exceed 10 minutes. Repeat procedure if required. Some stone & concrete may change colour after treatment.