Re:
Polish = The progressive use of finer abrasives to remove blemishes until the desired finish is obtained
Buy a pack of assorted sheets of Aluminium Oxide 'sand paper' from B&Q. Select one of the coarser sheets and cut it into strips about 25mm wide. Secure the fork leg and wrap the abrasive strip around the damaged area. Pull on alternate ends of the strip back and forwards until all trace of the corrosion has gone, moving to finer grades of 'paper' as you progress. If the corrosion is too deep in places (photo 2 ?) then use a file in that area and blend in as smoothly as possible
If you have a Tool Shop in your area, it is preferable to buy 25mm wide fabric backed abrasives off the roll in the grade and length you require. You can then put more effort into the job without the strips breaking
To complete the job you really require a ' 2 pack ' etching primer as a base to your paint finish