I use f-13 on all my gutter jobs. When I first started using it I had to play around with the water/chemical mix. If you mix it too strong it will take the paint right off. I had that happen once in a small area of facia i tested. With a 15-1 mix I have found that a few minutes dwell time and a rag to wipe or brush to scrub, they come 100% clean. The key is not letting it dry on the gutters. Keep it wet and do small sections at a time. That how I get it done. Everyone finds their own methods that work for them.