Before you start painting the door, determine if the stained door is sealed and when exactly the door was stained. If the stain is not protected with a sealant and the door was stained less than two weeks ago, wait for the stain to cure.
When the stained door is ready to be painted, sand it with 200- to 400-grit sandpaper. Be sure to sand the entire door lightly. However, at the corners, you will need to sand a little more vigorously.
Unless the stained door you are painting is made of flat pieces of wood, you will need to use a wire brush to vigorously rub over irregular surfaces.
Use a piece of tack cloth to wipe the door. Avoid using a cleanser or chemical as it will not let the primer adhere onto the door.
Cover foreign objects such as the door handle and latches with painter’s tape.
Use a paintbrush to apply a uniform coat of oil-based primer onto the stained door, you are planning to paint. Allow the primer coat to dry completely. It should take about two hours. When the primer coat dries, sand it using the same sandpaper as you had used before. Clean with a piece of tack cloth.
Apply another primer coat and let it dry just like the first primer coat.
Use roller for painting large, flat sections of the door and an angled paintbrush for panels and details. Continue until the entire door is painted. Check for unpainted spots and apply another paint coat if required. Be sure to allow enough time for the first paint coat to dry completely before applying the second paint coat, if required.