Stop wasting your time. The brown stain doesn't hurt anything. When I first began using my WSM, I was anal about cleaning my grates to shiny nickel new every time. It was 30 minutes of elbow grease in the sink after every cook.
After lamenting about the work to get em clean on this site, others gave me the same advice I'm giving you: Stop the madness! Just burn off the residue either by putting them on the coals after your cook is done, using propane torch or weed burner, or as I do now, put them on the gas grill on high for 10-15 minutes and then use a brass grill brush to knock off the charred chunks. After a year and half of this method, my grates are black, but not rusted. I spray em with PAM before each use and the meat doesn't stick.