I wouldn't paint it. I'd hire a professional power washer company. Definitely don't mess around with bleach. I tried it years ago and badly damaged my lungs with the fumes.

I googled power washing alum siding and came up with this. I think the printed version is below the video. It is very important to wash from above so water doesn't get in the seems.

Here is a link that might be useful: Power washing alum

There was a story in the Washington post that realtors in Maryland were calling the greyness a "patina" and using it as a selling point.

Here is a link to power washing VINYL siding. How to Pressure Wash Vinyl Siding

This post was edited by albert_135 on Fri, May 3, 13 at 14:27

Thanks everyone!! now a lot to observe & decide!