What are the best techniques for applying metal door primer to ensure a smooth and durable finish?

To apply metal door primer for a smooth and durable finish, follow these techniques:

1. Clean the surface thoroughly to remove dirt and rust.
2. Sand the door to create a smooth surface for the primer to adhere to.
3. Use a high-quality metal door primer and apply it evenly with a brush or roller.
4. Allow the primer to dry completely before applying additional coats.
5. Sand between coats for a smoother finish.
6. Apply a topcoat for added protection and a polished look.