shubydoobydo, looks like several ideas have been presented. If this machine was in my machine shop for repair, 120 grit emery cloth would be the choice. It is a good compromise between removing enough material (the parts are stuck together after all) and not being overly aggressive when used by hand. I have to respectfully disagree with the posters that see an uneven finish, especially when the emery cloth is used with the penetrating oil as a lubricant. Use 240 grit if you want to be extra cautious. The result will be the same, but will take more effort. 400 grit , fine steel wool and CAP have their place, but are not aggressive enough to help in this case since the parts have seized together. It is true that a polished surface will provide some degree of corrosion protection, but it will not help much in this application. The corrosion solution is to keep the clamp shaft lubricated and separate the clamp from the base when in storage. Fluid Film is a good corrosion preventive product, but can be a bit difficult to find locally and doesn't have the load bearing protection of even a light grease.
If it helps any, the above comes from 40 years experience in metals related manufacturing.