I haven't done it myself. But if you watch American chopper and a few other programs on Discovery. They show that after the tank is finished that it get bondoed. (Know I spelt that wrong sorry.) Then sanded and painted. Hence the perfect finish and apearance.
As to just a pure metal tank that has only been clear coated. Hammered with forms to get all the major dents out with lighter and lighter hammer blows till the surface is almost perfect. Learnt that at my first black smithing leason. Their are still imperfections. Just they are harder to see. You can then go back and file or sand the surface to get it smooth.
You can check out Lindsy publications. Belive they have a few books on the subject.