The dark area has not oxidized from the suns rays. I would let the homeowner know that it will look the same. There are some ways to “diminish” it and remove much of the oxidation, but it would be a huge upsell for most of us. That contrasting of a line would be near impossible to eradicate.
I doubt that it will ever match the rest of the siding. I’m with Crabby on this one. Inform the customer of your limitations and why you can’t promise any miracles. If it cleans right up and looks nice then you’re a hero. If it doesn’t (it won’t) then you’re the honest pressure washing guy that didn’t BS them and let them down.
Homeowner should paint the old shutters or replace them. 
Are you sure it isn’t just dirt that got trapped behind the shutters? That’s what it looks like to me. Might clean up pretty nice.
I thought that as well but then upon reconsideration I am willing to bet that if that house is any more than 1-2 years old there will still be a distinct difference. If it’s new I wouldn’t expect that much debris behind the shutters anyhow.
Of course, I could be way wrong. I hope it works out like a dream for the guy but I’d be more concerned with tempering expectations at this stage.
I new on here and only been washing house for a few years but this year I bought a soft wash system and have gone full time just this year so I do appreciate having a resource, So thank you…
I do feel I have managed the home owners expectations but wanted to make sure. I have read on here about the oxidation and don’t feel that there is a budget to fix it. I did suggest new shutters. Some Of this is dirt and there is a difference to be made. My instincts seem to have been right and now I’m more confident.
Best Regards
