You don't need to re-Nickel the front sight after you file it. I'll tell you a secret I've been using for many years.

After filing and making sure the height is right WITH the loads you intend on shooting the most, use a Dremel with a flannel wheel and some Flitz. High polish the raw steel and it will look just like nickel in a minute or two. DO NOT run the Dremel too fast as you do not want to over heat the good Nickel. After you finish you won;t be able to tell where the Nickel ends and the raw steel begins. Take you time, go slow and it will be so smooth and polished that even though the the top will be raw steel it should not rust.

Make SURE you use Flitz - it works much better than any other of the other polishes I've tried for guns.

EDIT NOTE: Sorry - I meant Felt - not Flannel.