hi there i'm about to sign up to doing a course, if your applying weed killer and your doing it for money or it formulates a service you provide for money (power washing included) then you have to take the two part course in every instance. As for type of weed killers there are two types we can use, contact and residual. see below net net you can get a weed killer that will lay dormant in the ground and work for upto a year. I had this done on a property by a gardener and i didn't have a weed for over six months, then once the substance degraded bang they just came back all at the same time! Incredible stuff and it really works but is very expensive, a small bottle that'll do about 350 metres costs over 80 quid! you could offer a guarantee that they'll have no weeds for six or more months, someone on the forum already does this!
Contact weed killers only kill the weeds they are sprayed onto. Although sprayed weeds will die, new weeds can grow back immediately afterwards. Many contact weed killers contain the chemical glyphosate and include well known brands like Roundup Pro Biactive and Asteroid.
Residual weed killers give longer lasting weed control by leaving a chemical residue in the soil or substrate which continues to kill weeds after application for a number of months. The more persistent products may only need one timely application per year.
Mixing a residual weed killer with a contact weed killer can give impressive results as the contact weed killer gives rapid knock-down of weeds, whilst the residual weed killer gives persistency of control.