Then you have to charge more for doing this. When our window cleaner does ours I can look out and see clear glass - I don't give a monkey what the plastics like. I wouldn't pay more to have the plastic cleaned, anymore more than if the guy cleaned the stone cladding.
You may not care about your plastics, but most custies do want the frames cleaned and expect it to be included.

Think you will find that most of us who include the frames as being part of the window....which they are....... have allowed for this in pricing. We have never yet had a custie say I just want the glass cleaned and we would never offer this service anyway.
Anyway, why do you employ another WC to do your own when you could do them yourself
