I just recently read somewhere you can use 1/4 baking soda and peroxide to a paste like consistency will take rust and cooking discolor off pans. It might work for your knife too. Worth a try and most of us have that stuff already at home.
If you have fine rust spots on the blade a #2 pencil will rub the rust right off. The graphite will chip away at the rust until its nice and smooth again. What you get back is going to depend on 1. The quality of the blade to begin with. (I'm thinking low as its apparently rusted) and 2. How much time you are willing to work at getting it cleaned up. Posted from my iPhone/Mobile device