If you're trying to remove rust stains, try a Davis product called FSR - Fiberglass Stain Remover. Its active ingredient is oxalic acid, which is pretty mild. FSR is a blue gel that adheres pretty well to vertical surfaces. Several applications may be needed, since it's intentionally not very aggressive. And, it's kind of expensive. Another source of oxalic acid that's cheaper is BoatLife TeakBrite powder. You can mix it with a small amount of water to make a paste, which will stick to vertical surfaces. Make a thick slurry, brush it on, let it work a while, and scrub it off with lots of fresh water before it dries completely.