I would go with a base coat and then clear coat to match the rest of the car, one can should be enough for the base coat because it's just for color, the protection is from the clear coat so you'll need multipe coats (and a clean area to apply it). Since it's just the fender it should be fairly easy to make it look good, a rattle can paint job can look fine if done properly especially in a small area, just make sure not to get any runs and then wet sand the clear coat to get rid of orange peel (if there is too much orange peel, some orange peel will look like stock and match the rest of the car, so it depends on what the final product looks like), I would probably go with 600 grit wrapped around a carpenters pencil (don't use your bare hands it won't look good) or a real sanding block if you have it, then move up to 1000 then 1500 grit, on such a small area you could use george bushes favorite pen to color the whole fender and then sand just until the pen ink is gone (real painters use a special paint or a really light coat of base color) it's not needed but it helps, then I would take it too a detailer and have them finish it, unless you can compound it and polish it your self. It will look very stock when your done, but the rattle can color probably won't be exact anyway, I would take the gas door off and bring it to a local paint shop and have them mix up some custom cans, the color will probably be more exact if they can copy the actually paint from the car instead of just looking it up in a database and then mixing like that.