Hammertone paint looks great IMO. If it is rusty, use a wire brush and sand paper, if it is undercoated or painted, use paint stripper. Rust Oleum hammertone silver is my vote.
Did mine last weekend while upgrading my fuel pump. If your tank only has a little surface rust like mine did a 3M medium grit sanding sponge works wonders. It does well on all the uneven surfaces. I used Plasticoat cast aluminum rattle can and it turned out great.
what ever you do don't sand blast it lol KABOOM! it may sound like common scence but believe me it's not. I've had many people come to the shop and want us to sandblast a gas tank full of gas.