Are you sure it is stainless steel and not just chrome? You want to be careful that you know what kind of metal you are dealing with so that you don't use something that is dangerous to one type and safe for another. Most bumpers are chrome and not stainless steel.
For the most part it is very difficult to remove scratches from stainless steel or chrome. Paint scratches are easy since the paint was applied and easy to polish. Steel is not paint and is not going to polish up like it. You can use a variety of polishes to remove waterspots and rust, but they typically won't remove scratches.
How deep are these scratches? Do you know what they were caused by? Pictures might help give some better suggestions.