Hi I just want to enquire to see how to use weedkiller? First I have a round up spray drum but someone mentioned to me I can buy a refill of weed killer exactly the same at less that half the price and pour in into the round up sprayer. Secondly when putting the weed killer down do I need to take the weeds out the first time then spray the area. Also can weedkiller be put over flower beds a few days before planting plants?