I have found that when cleaning gutters, the black streaks in particular need to dwell in the cleaner for a minute or 5 to loosen them up so they can be easily brushed clean.
Seems easier for me to apply the product to the gutters with a sprayer and then brush them,...instead of applying with the brush. Just seems like too much extra brushing,.... at least for me.
For those of you who have a compressor as part of your rig, you can use 2 gallon pump sprayers with a tire valve installed to add the air instead of pumping.
Get one of the air chucks with a trigger and a gauge built in so you don't over do it with the air.
You can also use spair air tanks if you haven't a compressor.
Be careful if you use an air compressor, the tank can blow up. Adjustable Pop-off valves are also available. An aquantance used a compressor to pressurize his weed sprayer, when it blew up, he got chemical all over his manly parts. no one cared to ask if his root died.