Prepare your surface. Sand the door thoroughly with medium-grit sandpaper to remove grime, imperfections and to roughen up existing paint or varnish. Vacuum the surface of the door, then clean with all-purpose cleaner and a damp cloth. Be sure to remove all the sanding dust. Place a drop cloth on the floor in case of spills, and tape a piece of plastic over the door knob and hinges to avoid getting paint on these surfaces.
Prime your door with magnetic primer. Available at home improvement stores, magnetic primer will be the undercoat of your magnetic chalkboard door. Have the paint department shake the primer in their mixing machine. Paint three coats of primer on the door; let the primer dry to the touch between coats.
Apply chalkboard paint to the door. Also available at home improvement stores, chalkboard paint comes in black and green. Paint two coats of chalkboard paint; let the first coat dry before applying the second.
Allow your door to dry for 24 hours before writing on it with chalk or adhering magnets.