Is it already painted?
I had one that was already painted - went over it good with a wire cup brush to knock off any loose chips/flakes and to rough up the surface for a fresh coat of paint. Put on two coats and was good to go. Had it for a few years and the paint stayed on very well, even through some brutal duck hunting and ice.
If it's not already painted, you may have to use some etching primer...which is kind of expensive.
Here's a very good link to some info. that will help you:
Parker Paint is a good one to use (what I used):
Check out the reviews on it too.