Agree with above - weed as normal then a mix of white vinegar, salt and a little washing up liquid so it sticks - I do mine in one of the 5ltr spray bottles from Amazon. You do have to weed first though because just spraying the leaves wont work. Refill the cracks with a resin based filler it's awesome and really easy to do - you mix it with sand and sweep it over the top then rinse