320-360 is what you need for paint prep if there are scratches. If you're just roughening good paint for a recoat then 500. Dry paper for paint prep, wet sand for correcting paint defects, 600 then 1200 and polish. If you see bare metal then you need a light prime coat, SEM makes a nice self etching primer in a rattle can.
Rustoleum is fine, it's just soft and well wear pretty fast. Acrylic enamel with a hardener will perform almost as good as powder coating, and doesn't chip like the latter and can be sanded and recoated.
If you don't have spray equipment then the rattle-can enamels have to make do. Use a couple of mist coats and plan to do it again after some use.
Another solution is to remove everything you want recoated and drop them off to a powder coater. They will sand blast and apply a new finish, color and texture to suit. If you shop around you should be able to get the two trellis pieces and your guard done for about $100, maybe less. If you sand prep them a body shop might shoot them in satin black for you with automotive enamel for the same price, maybe a little less.
2009 V - custom black-on-black - SOLD