Minor Pitting
You may be able to live with this if it's not on the shaving edge or you can usually take some time to sand it off as described in the Rust Removal page.
Major Pitting
You'll need some mechanical help or a large amount of patience with this level of surface pitting. Normal hand sanding would take a long time. You may want to research into getting the blade reground by someone on the forums who has the ability to do so or even trying to DIY with a rotary flap wheel.
You can get good results with a Flap Wheel if you have patience and are careful. Once you've removed the pitting to your satisfaction you'll need to do the normal sand paper progression to get a good finish on the blade.