Chris,
What do you feel is the challenge with this vinyl? Yes it's nasty but if it's organic you can clean it. Start with 1% , let dwell and rinse. Bump it up if you have to.
Hank
I would use caution with that type of siding. It looks like it may be Naillite brand rough-sawn shakes. If so there has been many issues throughout the past years with this siding. The homeowner may not be pleased with what's under that filth and may fall back on you. Not to mention it may crumble into pieces as soon as it is touched (if it's from the 90's). If you have documentation stating that it is another brand, you can probably disregard this warning.
Maybe do a test spot first in an inconspicuous and get approval from homeowner. Man if it cleaned up nice, can you imagine what a killer before/after that would be???
Totally subscribing to this post. Take lots of pictures!!! :)
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Brandon Vaughn
All-Clean! SoftWash Gresham, OR brandon@allcleansoftwash.com www.allcleansoftwash.com 503.887.6404