Ther are several products that can be used to do this. Brownells make at least two and can be found here.
http://www.brownells.com/aspx/NS/sto...d+BLUE+REMOVER
and here
http://www.brownells.com/aspx/NS/sto...le=STEEL+WHITE
Beachwood Casey makes a product also. It can be found here.
http://www.brownells.com/aspx/NS/sto...SHING+PRODUCTS
Scroll down it is the 5th product down the page.
I have used the Beachwood Casey product. I used steel wool and fine sand paper to improve the finish on a gun in sad shape. I have not used either of the Brownells products but I have had very good results with other products they sell. There is an 800 number at the bottom of the page and they have always been helpfull to me.
Hope this helps
Dan