Hello, you can use this product with a brush to treat black spots on tile grouting. The product is a liquid suitable for surfaces such as grouting, and applying it with a brush dipped in the product is a practical method for precisely targeting the affected areas. Make sure the room is well ventilated after application, as the product contains chlorine and gives off a strong odour. Make sure you wear suitable protective gear, too, as it can irritate skin and eyes.