When you first start out you will struggle to get them spot on first time around, it's the learning curve thing
It is more than possible to get them right from the very first clean, and this gets easier as you become more experienced.
It's all very well to say, 'Yeah, course you can get it right, just scrub and rinse really, really thoroughly'.
But if you are only just starting out in WFP mode then it isn't as easy as it looks!
But it can be done, takes longer, uses more water and you usually have to go back over the windows a couple of times.
Ian