i wfp'd the whole outside of a unit the other day, was a big place, was knackered! lol
i dunked the brush in some lemon degreaser i had, worked well, i suppose it would be like traffic film remover! just some chemical i got given and he said try this
scrubbed the dirt off, and then rinsed...but i dont see the need or pure water to be honest, so if you can get to a tap on site i reccomend using it