Spray Nine and the good old hand scrubber. PW first to remove all grime and then start scrubbin'
I'd even suggest an organic vapor respirator...I have tried quite a bit of products over the years including some high dollar worthless products. I found that oven cleaner $1.00 a can at the dollar store removes thoose black streaks and stains on gutters makes them look brand new. Be careful if you do this you will need plastic gloves and a dust mask.
Yes youd better use one of those not a dust mask what was I thinking... but the stuff does work greatI'd even suggest an organic vapor respirator...