Tony is right, sand paper is your last resort.
After cleaning with any track cleaner you like, alcohol, or any other track cleaners. My next step would be a track eraser.
For stubborn corrosion I would try this.
http://www.modeltrainstuff.com/Walthers-Cratex-Abrasive-Block-XF-p/949-522.htm
Like Tony said if all else fails then use a very fine sand paper or emery cloth.
Be sure to clean your track again with track cleaner to remove and fine particles left from the eraser and or abrasives. I like to use a hand held vacuum (my new favorite tool) before and after cleaning.
To prevent corrosion I would use a product like this for the final cleaning.
http://www.modeltrainstuff.com/Pacer-Technology-LLC-Rail-Zip-p/ptc-pt23.htm
If you ask one of the model railroad experts they may know of a cheaper product that prevents corrosion.
The reason you want to stay away from harsh abrasives is that they can leave behind small scratches in the track that will collect dirt and could cause a loss of electrical connectivity between the locomotive and the track. If nothing else a scratched track will require more maintenance then a track that is not scratched.
If it were me I would use track with minor scratches that I already have rather then I would buy new track, I don't mind extra maintence.
Let us know how you make out.
If I have not already said so; Welcome to the forum, glad to have you onboard!