Just wondering are you using hot water? I personally use the 2 step method and almost never use a brush on trailers. One of my accounts has black tractors so I do brush them b/c they don't want acid on them I just use a standard 14" brush I bought somewhere on the internet, probably the cheapest I could find good luck.
2 stepping is appling acid to the truck followed by a soap and then rinse. I don't have time to go into alot of detail right now but do a search there is alot of info on 2 stepping here.
Jerry B.
Cleaner Image Powerwashing
Hortonville WI.
920-740-6100
www.cleanerimagepowerwashing.com[/url]