Sand your door to remove the finish. New paint will not adhere to paint or stain, so sanding the door will give the paint something to hold onto. Wipe away dust with a clean cloth.
Use painter's tape to protect the door's hardware, such as hinges, knobs or a knocker. Apply the tape so that it fully covers the hardware but does not stick to the door.
Apply a base coat of exterior latex paint to the door. This is the paint color that will show through after you apply the crackle medium, so remember that this shade will not be the dominant color of your door. Allow the base coat to dry completely.
Apply the crackle medium to the door. Use long strokes with your paintbrush that all go in the same direction. A thin coat will create small cracks while a thicker coat will create larger cracks. Allow the crackle medium to dry completely.
Apply a top coat of exterior latex paint. This color will be the dominant one for your door, with the base coat color showing through in the cracks. Allow the top coat to dry completely, then remove the tape from the door's hardware.