Yeah, what your doing is an initial clean. Not in the builders clean sense obviously but I would have a scraper blade (for the paint specs and ronseal back spray) as well as a dish brush and even a tooth brush when doing the first timers. Your WFP brush won't have the agitation required to remove a year or 2's worth of crud, spiders nests, traffic film and bird gunk. Personally I don't charge any extra for the time involved in them, I love cleaning glass and it blows the comp out of the water and might even inspire some semblance of custy loyalty.