Black Streaks On Face of White Rain Gutters

The streaks are normal dirt carried from the asphalt shingles on the roof. It took me about two years to realize I had a blockage underground. Anyway... I'm left with these streaks on the face of the gutters. Spraying with 409 does fair but wiping it with industrial strength bleach, then 409, then bleach again does the best and nearly removes the streaks completely.
Anyone have a better suggestion? The gutters are aluminum with - I presume - a powder coated white surface.
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409,
presume -

I think it is both dirt and mildew, at lest it is on my home. I got some stuff at Home Depot call "Krud Kutter" http://www.dtep.com/supreme.htm . It did a great job, but I got a little carried away. I kept wiping and removed the paint! I ended up painting the gutters with exterior latex pain. After thinking about it, I came to the conclusion that a quick wash with some TSP and bleach followed by a coat of paint with a mildewcide added was the quickest way to solve the problem.
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