is it best to spray weed killer a few days before you clean so it is absorbed through the weed to root or after cleaning but before sanding,or will the weed grow no matter what you do with air bourne spaws landing on the sanded joints,can you get away with just cleaning and sanding.Are there any weedkillers you don`t need a license for?How come you don`t need a license to spray your own garden,as long as you use common sense and stay away from flowers and pets etc an wear a mask is it really that risky?