My husband and I clean a lot of siding for our customers. We've always preferred to clean vinyl siding because you don't have to worry about any oxidation like would happen with aluminum siding. Even with vinyl siding, comes aluminum gutters and soffets and it's been our experience that white gutters tend to stay dingy after cleaning, whereas other colored gutters such as yellow, blue and brown, will clean up very nicely. Why this is we do not know.
When people call us to clean their siding it is usually because the north side of the house has mildew on it. This can be black or it may look like grass stains on the siding. Or it just may be plain old dirty. Often when we clean it, it may be the first time it's ever been cleaned.
For the most part, vinyl siding will clean up. The only thing that won't clean is when small black bugs are on the siding, or the residue of the bugs. Oftentimes one can't see the bugs until one is up close. We've scrubbed them with a hard brush, soft scratch pad that won't scratch vinyl, or sometimes just used our fingernails, gently. But the black top surface comes off, leaving a lighter brown color behind, which will not budge.
Until recently, we've never cleaned any aluminum siding without having the color look spotty from the effects of oxidation on the aluminum. When years have passed before aluminum siding is cleaned, be expected to paint afterwards. Even white paint may look dingy or even have a yellow cast.
This past week, we cleaned someone's home that had no oxidation whatsoever. What a change that was. Come to find out the homeowner usually cleaned it once or twice yearly, herself. She would have done it herself this year, but she's had health issues to deal with instead.
From her experience, I see where if you want to maintain the exterior of your home properly, you should clean it yearly. This would also help to prevent the bug buildup. Oxidation is a fact of life, one cannot change it's effects on siding any more than one can change a cut up apple from turning brown without using lemon juice or similar topping.
Both painted surfaces and aluminum will oxidize. It's just that if your home is cleaned, the oxidation will be kept off the surface, and it will weather evenly. Eventually it may need to be painted, or treated once more, but if it needs to be cleaned inbetween times, it will not look like it's falling apart.
Our recommendation is that you not use a powerwasher as that can actually harm the siding if the pressure is too hard, it can remove the vinyl surface off the window frames, and the water can get underneath panels of siding and harm your insulation. Cleaning by hand is the safer way to go.
If you as the homeowner choose to clean your own siding, check out the articles linked below to help you decide how to proceed. Make sure whatever method you go with that you get the windows cleaned the same day preferably to prevent chemical spotting, or staining on the glass.
Like anything else in our lives, prevention and upkeep may not be easy, but it will benefit your home and your pocketbook long term.
http://www.ehow.com/how_5204166_clean-vinyl-siding-much-scrubbing.html
http://www.ehow.com/how_5323362_keep-aluminum-siding-oxidizing.html
http://www.ehow.com/how_5215872_keep-damaged-after-cleaning-siding.html