I found an old metal fire extingusher from the 90s. How do I clean it so I can store drinking liquids in it?

The fire extinguisher is old and was full when I found it. I've emptied all the contents and now want to sterilize it. I think it'd be cool to keep pepsi in or things like that. How do I clean it to drinking potential? It's a bit rusty on the outside but most of the paint is still there. It's pictured here http://en.wikipedia.org/wiki/Fire_extinguisher and contained sodium.