Is the rust from the air conditioner? or from the soil? Maybe bleach solution would work or lemon juice.

You might also try *Wink* for rust stains. Also works on clothes. It comes in a small liquid bronze color bottle where laundry and cleaning agents are in you grocery store.

Wink works well on rust stains. You can check your plumbing aisle for rust removers. BarKeeper's Friend & Zud are powdered cleansers that contain oxalic acid, which is the ingredient in most rust removers, to remove rust.

I know this is an old thread, but I found something that worked for me. I used Iron Out. It took a couple of applications with some scrubbing with a terry cloth, but it is clean. In the future I won't let it get as bad...so I hope it comes off easier.

Here is a link that might be useful: Vinyl Siding Cleaning - Fast and Cheap