Go to your local co-op and buy tank primer and tank enamel,it is formulated for that application.
Clean the tank with a stiff wire brush and pressure wash it with a fungicide killing agent soap.
Let it dry,primer one day and paint the next day.
I painted a lot of tanks in my day when I worked for a local co-op.
^Spot on 100%
Our family owns somewhere around 50 propane tanks ranging from 100 gallons to x2 100k gallon tanks used to sell propane on the side. I’ve painted many, and the final painting process is about 5% of the work.
For a decent end product don’t be afraid to get real aggressive with the brushing and cleaning of the tank. With primer less is often better. If you have too much primer that takes longer to dry, you can have more issues in the long run with longevity of the paint coat. Humidity is your enemy. Most of the one’s I’ve painted/cleaned were hung from our lull outside on dry warm fall days with a slight breeze.