Ok ... I'm just a little homeowner, not a contractor, but I AM WOMAN HEAR ME ROAR! LOL I figured out how to get Rust stains, that were left from my wrought iron furniture, off my Trex (2nd generation) light gray composite deck planks ......READY???? A simple and cheap Hydrogen Peroxide and Baking Soda paste you can make, and an old toothbrush! IT WORKS! (I came to this site looking for a solution or recommendation and didn't so I started trying different things myself and WHOLA! Hope this helps! Each stain took me about 10 minutes, to scrub with this paste, but they all came up! Good Luck!