Avoid trying to put heavy coats on. You want to apply many light coats with very light sanding to smooth and level the paint. Practice on a sheet of metal and if you paint a section with the tank shooting it first you can know what too much paint will look like and understand when it runs. Start at top and work your way down, overlapping your coats. Consistent distance, speed and spraying past both ends of the tank are needed to get best results.
Your last one or two coats should be heavier wet coats. Once the paint has hardened in a week I would color sand with 2000 wet dry. Once you have a nice level very clean looking paint I would top coat with SprayMax 2K Urethane clear and polish it after cured for a few days. 4 coats recommended, 3 minimum...not counting a dusting first coat to begin the application.