When a metal object sits on a humid rug or even the carpet in the trunk of your car, it often leaves rust stains. They're very difficult to remove and there's always been only one good way to get them off: Oxalic Acid -- also known as Lemon Salts. So for years I've lived with the frustration that I would recommend this to people, knowing they'd have difficulty locating any. The problem is its name. Lemon Salts sounds like something you would find in the spice section of the grocery store. But it is a dangerous acid. So instead, it is sold at the drugstore, and often kept behind the counter. You have to ask the druggist for it. Why, because it is too easy to confuse with food. Although it did remove the rust stains better than anything else, it also tended to remove a bit of the tissue as well.
I was delighted when I came across the RUST GUN, a non-toxic rust remover that doesn't eat the fabric you're working on. Unfortunately it is a bit hard to find. If you know of other rust removers that are kind to textiles, let me know.