Before painting your door, double check to make sure it is smooth, clean, dry, and dust free. If the door was previously coated with an alkyd or oil-based coating or you are unsure, make sure to prime it with a high-adhesion waterborne primer prior to coating with a waterborne paint.
Start by covering the floor beneath the door and taping off or removing the doorknob. With the door open, use a brush to carefully paint each edge of the door. If you get a little paint on the hinges, use a damp cloth to wipe it off while it is still wet.
For smooth doors, a small, short nap roller can be used. Load the roller with paint and roll from top to bottom, from one side to the other, until the entire door is coated. As soon as the door is coated and before the paint has had time to dry, roll back over the entire door starting each stroke at the top of the door and rolling all the way to the bottom to create a uniform finish. Using this method will leave a fine stipple on the door, making it much easier to touch up in the future.
For paneled doors, use a brush to paint each recessed area first. After each recessed section is painted, wipe the excess paint from the brush, and feather out any paint that has overlapped onto the broad surfaces to prevent hard edges. Once all the recessed areas have been painted, brush around the doorknob and any door hardware. Next, brush out the broad surfaces from the top to the bottom of the door until the entire door has been coated. For doors with wood grain, brushing in the same direction as the wood grain will help minimize the visibility of any brush strokes.
Remove any tape immediately after painting while the paint is still wet and re-tape before doing additional coats. Allowing paint to dry on the tape will make it more difficult to remove and can even peel away some of the fresh paint with it. If you removed the door hardware, wait until the door is completely dry before carefully reinstalling. If future touch ups are necessary, use the same type of application tool as what was used to paint the door.