A couple of days ago I found this thing on the to the park. It is quite heavy and all rusty. I found a couple of methods how you can get it off:
Vinegar naturally cleans many materials, including rust-covered metals. To remove rust from metal objects (coins, silverware, jewelry, etc.), soak in undiluted vinegar (the standar 5% solution available in grocery stores works just fine) for as long as necessary before simply wiping or scrubbing the rust from the metal.
Baking soda is another natural cleanser that removes rust. Create a thick paste by mixing baking soda with a small amount of water, then spread the baking soda paste over the rusted area and allow to sit before scrubbing to remove the paste and the rust.
Some people recommend commerical soft drinks like Coke to remove rust. Apparently, the citric acid in sodas like coca cola works away at the rust on metals, tile, you name it. Just soak a cloth or paper towel in some coke, then rub the rusty area to remove the rust.
And so of course, plain citric acid will also remove rust, especially from steel. You can buy a variety of commerical cleaners with solution of 6% citric acid at most grocery or drugstores. To remove rust, simply soak a cloth or paper towel in the citric acid solution and scrub the rusty area.
OTHER METHODS:
Scrape and sand away rust from the surface. You can use sandpaper, a wire brush, or crumpled aluminum foil to scrape off the surface flakes of rust. This will leave the surface underneath clear and ready for repairing to prevent more rust. For rusted surfaces to big to scrape by hand, use a wire brush attached to a drill, or an industrial sandblaster.
Read more: http://wiki.answers.com/Q/How_do_you_get_rust_off_of_metal#ixzz1IgF6krCP
I choose the last method in the end because natural rust removal sounds a bit too extreme for me and I didn't want to get my bath damaged doing it.
So I took off the rust using drill with wire brush in the end. It is a very messy and hard work but quite quick. Actually I find it quite hard for a girl to do, as the drill is heavy and need a strong hands to keep it on one place. Afterwards I spayed it with rust preventing stuff and painted it.