Wow. That's a project. :)

Start by sanding the entire surface to your preferred smoothness.

1. Begin with #120 grit sanding block to remove the majority of texture. Finish sanding with #150 grit sanding block.
2. Fill voids with MH Ready Patch Spackle and allow to dry. Sand smooth with #150 grit sanding block.
3. Prime the Ready Patch with your preferred paint and allow to dry.
4. Roll paint using a 4" Wooster 3/16" nap roller cover or comparable quality roller and cover. Be sure to lay off paint using vertical roller application.

Option: Remove the door, prep it same as above and spray paint it with two coats of Krylon Colormaster Exterior Paint.

Thank you so much brushworks!!! This is the advice I was seeking!

You're welcome. Let me know how it turns out.