The black streaks in gutters are in the oxidation layer of the paint.
F-13 removes the oxidation and in turn removing the streaks and unlike butyl-based products that when diluted with water lose their strength rapidly, F-13 is effective at dilution ratios of 50-1.
F-13 can be used safely at dilution ratios as low as 5-1. 20-1 dilution normally removes the oxidation with light agitation, making F-13 very economical to use. F-13 works best in temperatures above 60 degrees.