I guess it depends on whether the brown is oil staining, or rust....
If it's oil staining, try a rag soaked in a degreaser - here in the UK that might be Gunk or Jizer. If you're careful, you could also use petrol or diesel (quite expensive though, and petrol of course is a major fume & fire hazard). Rub the affected areas until clean. Use whatever chemical works the best. If you're not going to paint them, then lightly oil the surfaces afterwards to prevent rust.
If it's rust, then you need to break out the wire brushes. Brass brushes would leave less marks than steel brushes, but will wear out much faster.