It depends on the condition of the paint, but in most cases, I always start with something fine, like 3M perfect-it rubbing compound (kit scratch out works pretty well too and is cheap!), and if that doesn't produce the results I'm after, I start over.
If the paint isn't too thin on top, (or severely sun faded) you can probably wet sand starting with 1000 grit and work your way up to 1500 or so, then use the fine rubbing/polishing/finishing compound of your choice.
The caveat to wet-sanding is that if the paint is thin and you go through, you've made things pretty ugly and there's no going back. But on the flip side of that, you can get out old fuel stains pretty effectively if you can pay attention to what you're doing, use new paper in clean soapy water, and not go overboard.
An alternative would be to make a bigger investement in finishing materials and spend the time polishing it out. I use a cyclo dual head polisher-
http://www.cyclotoolmakers.com/polishers.php 3M makes good stuff, and so does Meguiars in my opinion. But whatever you decide on, use that whole system- 3M in particular. They spend a lot of time engineering their products to work together, and so do other companies.
With some attention to detail, it's pretty easy to save a tank like you've got. Making them shine again is always a fun project, good luck!
