Good question, and one we all face now and again.
If you have one, a rotary wire brush, a brush on an electric grinder, is a good place to start. Don't go doing this to some museum piece that has intrinsic value by virtue of its age and condition, but for a knife that is just neglected, and one you want to return to service, that is a good start.
And whether you have a rotary brush or not, the next step (or the first step if no grinder) is Scotchbrite -- that scratchy pad stuff you probably use to scour dirty pans. It can be used like a pad on the knife or one can put a pad on a flat surface and rub the knife lengthwise on it. I recommend doing it lengthwise because it leaves a nice fine scratch pattern all going in the same direction -- something we do on handmade knives to make them look jazzy.
You can also start with abrasive papers on the blade like we do in hand finishing a knife, start with something like 100 grit and work down to 600 or more. This is tedious, but that's how I made those blades in the pictures look so good -- went down to 1200 grit
Don't expect any of this to get out the deep pits or deep scratches. They are in there for good.
Hope that helps. It all involves a lot of elbow grease, but unless you want to have the knife reground, it's going to take hand work.
