Combine soap and water in a bucket. Adding an ounce of soap to a few gallons of water is sufficient. Wash the screen door with a scrub brush and a sponge. Remove as much dirt and powdery paint as possible. Spray the door with a hose, starting at the top and working your way down. Let the door dry.
Cover the ground around the door with newspaper or a drop cloth. Make sure that the threshold is protected from paint spatters as well.
Place a strip of tape around the outside of the door. Put tape on the edge of the screen as well. Do not paint the screen, as paint will clog the mesh.
Pour primer in a bucket. Add a pint of paint thinner for every gallon of primer. Mix the two together with a stir stick.
Brush the primer on your door with a flat brush with natural bristles. Start painting at the top of the door. This way you will catch any drips as you tackle the area underneath. Allow the primer to dry overnight.
Paint your finish coat with a synthetic-bristle brush. Keep brushstrokes smooth. Two even coats of paint are better than a single, thick coat.
Carefully remove the tape. Take care not to peel off any of the paint in the process.