hi ian i have the same problem as i do a few dairy farms, house and conservatory covered in fly poo, i pre- spray all frames with a chemical i use (about 4 windows at a time )and when i go back to it you can see it melting all the poo and leaves no stains bring it like new again ,it never works with just water it takes not that much longer to spray windows it truly is amazing, i have been using it for about 7 years since i first discovered it and there is nothing that comes close i promise you ,once you use it you will use it all the time ,i use a 7ltr hoselock spray bottle ,i also use it on con roofs and gutters ,speeds you up as there is a lot less scrubbing involved if you are intrested email me and i will give you details of where to get it . it comes in 5ltr or 25ltr ,i get 25ltrs a time because i never want to be without it ,it is a must have if you have this problem
good luck antony
antony_platt@sky.com 07837247410 or call me