You need to use an injection sprayer in conjunction with a reasonably powerful pump. If you've got anything less than 250psi and flow rate of less than 3 gpm forget it, the sprayer simply won't work properly. Basically if you have a portable it won't work unless you have one with a very powerful pump. They are a truckmount accessory.
Also forget the adjustable dial type ones, they are not accurate in the slightest so you don't have a clue what you're putting down. The fixed ratio ones are the only viable option, they are normally about 1:8 dilution so you just mix up your stock solution in the container 8 times stronger than normal.