open the hood, sand it til the rust is gone, 3000 grit should do it, put some touch up panit down on it. atleast a color that is close, doesnt need to be exact, unless you want it to be perfect looking.
ive done that to a few spots on my truck, i hit it with 2000 grit, 3000 grit, then put some touch up paint, then 3000 grit again after it had dried, and then buffed it to damn near perfection!! theres no clear coat on it, but it looks 1000 times better!!
if you don't kill the rust and just sand it down and apply paint over it, the rust will just return and eat through the surrounding area. you can do what iceman has suggested, but i would apply a rust killer prior to doing so.