Already existing powder coated paint is coming off... has some rust near the bottom.
WHat is the best way to paint on it?

Sand and prep door. I like to use "Exo-Rust" or "Extends" This is a paint primer for the Rust spots. It actually changes the Rust at a molecular level and eats it. Then a Kills or Binz Primer/sealer Then the paint. Tint the kills primer the color of the door you are going to paint. Less coats.