[Lippyp]

I use a similar product called Deox-C, it comes as a powder and you mix it with hot water and soak stuff in it. It works amazingly well and will remove all but the heaviest rust in one 24hr soak. Its non toxic and non corrosive although it stings a bit if you've got any nicks on your hands.

I did these old metal shutters that were on my house in France. They'd not seen paint for decades and all I did was make up a temporary bath in a hole dug in the earth floor of the cellar lined with some plastic sheet, mixed the stuff in a bucket of hot water and left it for 24 hrs. The results speak for themselves. All I needed to do was take them out halfway through and give them a tickle over with a wire brush and drop em back in and then hose them off They came out looking like they'd been sand-blasted, all clean and ready for paint, even in any cavities. Heres a before and after picture.