Rust on your siding? Actually it’s not rust. This is most commonly known as hard water stains. This comes from hard water and mineral deposits leaking onto your siding. The source of this problem is usually a leak from your ac unit. Yes, it is unsightly and yes, your HOA may fine you for it. Thought you would put a little CLR on it and make it go away? Think again, we use a special chemical made just for hard water stains and mineral deposits to make your siding look like it did before it became a problem.