you are actually better off telling the customers it takes a few cleans,I always tell them the first clean is more about the frames than the glass and they always accept this especially if upvc, if the frames have not been touched for a long time then it will take 2-3 cleans to get the area totally sterile, as they will always leak very slightly long after you have gone but i always scrub the top vents out on a 1st clean as a long term strategy, if u communicate with the customers it always seems to impress and also demonstrates that your passionate and particular about your job.
on 1st cleans always frames,sills then glass,on the glass you need to really get up in to the top corners as this is usually a hot spot for dirt, if you decide like me to do the vents stop your flow as you go over them and if particularly dirty drop your brush down and take any crap out of your bristles by running hand up and down the brush with flow on, always allow the top windows to stop dripping before you clean bottoms.
maintenance cleans,I usually go once around the frames whilst now missing out the vents, 3-4 times across the sills and twice on the glass I usually work the brush in fan like action left to right on the glass and whilst controlling the brush by pushing down on it to stop it hitting sides of the frame,when rinsing I always work horizontally getting up into top corners and taking the brush side to side whilst dropping 1-2 inches at a time,if using a high flow keep brush close to the glass to absorb some of the splash back, buy the lightest most rigid pole you can afford and a 25-27ft one is probably the minimum I would start with as will get you over conservatories,most extensions and will allow you to do 3 storey work, if you can afford it a gardiners slx is the best choice but if your on a budget then go for the clx as it's a fantastic pole for the money,never brush with your arms alone alway try and lock them in close to your body and use your whole torso to move the brush, this is a great way to prevent repetitive strain injuries.
feel free to get in touch if u need any more advice, numbers on my website.
graham