Originally Posted by dirtslayer
Use a wire wheel on a grinder. Paint. Done.
Hey WRXGoBerserk, I agree with dirtslayer. I used to have a Jeep I used for off-roading that was constantly battling rust. Instead of using normal paint, may I suggest going to an autoparts store and getting a more severe rust protector (sometimes called "sound deadener"). Usually about 50% black paint/50% rubber. When I used to paint the undersides of my jeep to avoid rust, it would always bubble up and come back. But when I used the "rust proof sound deadener" it would never have an issue (unless I slid the underside over a rock; in your case a curb, steep driveway, improper jack usage... in that case you would need to re-apply immediately)
Hope this helps...
EDIT: forgot something, if you are worried about overall weight of your car... do not paint the entire underside! as you can imagine a full undercarriage treatment of this stuff can actually add a few pounds.