Following the recommended steps and using the White Knight Rust Guard 400g Grey SLS Etch Primer Spray Paint before painting is advised. Achieving the same colour in a spray can form is difficult as the paint on your door has been weathered and is no longer its original shade. I suggest using the Ozito Spray Gun to repaint your entire door assembly.
I have attached a video below showing how to use a paint sprayer. - EricL