From my experience of painting iron roof, you need to use Quick Rust Remover to remove rust first, then do some sanding if rust is not cleaned. Then clean carefully. Then apply Quick Rust primer paint twice. Then paint with top coat twice. All paint must be suitable for iron becuase iron change size with different temprature.
Quick Rust is a product from Dulux. You can buy it from Bunnings or Miter 10 mega.
For wooden door it would be much simpler. Just clean first and apply primer and top coat. Use wood paint.