i was always told Virosol if left on for too long stains, so i decided to use Ubik, once i got a few splashes onto a window without seeing it, and it did dry a bit dodgy, other than that never had any problems when using it cleaning conny roofs and certainly not on FSG. Just dont let it dry, i soak the area with pure first, then apply ubik, scrub, leave a few mins and then rinse thoroughly before moving on. Has worked for me everytime so far! Might try Virosol or some other tfr soon though as nearly out of Ubik anyway.