A3. This product contains a quantity of iron sulphate as a mosskiller and this may have caused the discolouration. For this reason, we do place a warning on the pack that iron sulphate can cause stubborn brown stains on clothes, paving slabs etc and that the product should be brushed off immediately if accidentally spilt onto such surfaces before it gets wet. A 50:50 mix of lemon juice and water, or vinegar and water may help dissolve the stain, please test in a small area first. If this does not help then rust removers may be purchased from hardware stores.