First Vinyl Siding Acid Wash

So, I have SH based house washing basically figured out. But, on my last house wash, the vinyl siding that was under an overhang, looked brown and stained. Soap, SH could not get rid of the staining.
So, I did my first acid wash, and the siding looks brand new! Here are some pics:

And here is the product that I used: Zep Calcium, Lime and Rust Remover.

I am sure there is a better product, but this worked like a charm. However, it did require some light agitation with my brush, and then rinsed off like new. If anyone knows of a similar product that does not need scrubbing let me know!

I did a short blog about it if you are interested: