HI Chris,
I have had this problem a few times.
I give the customer two choices
1) It will cost you this amount £??? for me to clean all the paint etc of the windows, then you’re windows will be perfect.
2) I will do one window per visit at no charge.
They only get choice 2 if they don’t take choice 1.
Again, yes this might be too soft, but I need to make sure the clients windows are clean as that is advertisement for me :-)
Andrew