When painting a flat door, the trick is to work quickly so the paint doesn't dry before you cover the entire door. This helps avoid lap edges, which occur when you overlap dry or slightly dry paint with wet paint. Although you can paint a door with just a brush, using a roller to apply the paint followed up by a brush provides the smoothest surface. When painting new doors or changing colors on a door, start with a coat of primer and follow it up with latex or oil-based paint.
Clean the door with soap and water to remove and dirt and grease. Rinse well and allow the door to dry.
Open the door and place a door jamb or piece of wood to keep it from closing. The door needs to remain open until it is fully dry.
Place a piece of cardboard under the door to protect the floor from paint.
Clean the hinges with rubbing alcohol and a paper towel. Cover them with two coats of rubber cement to protect them from paint.
Remove the doorknob, lock and any hardware from the door. You can mask the doorknob and lock and leave it on, but you'll get a cleaner edge if you can paint underneath it.
Remove any loose or chipped paint with a paint scrapper and repair any dents or cracks in the door. Sand any rough paint edges with 80-grit sandpaper, followed by 120-grit sandpaper. When finished, sand the entire door with 180- or 220-grit sandpaper.
Use a 2-inch brush and paint the ends of the door.
Paint around the hinges starting with the inside hinge. You will want to paint this side of the door first.
Paint the door starting at the top corner and working down. If using a roller, use a 1/4-inch nap roller cover. Move the roller up and down the full height of the door moving left to right (or right to left). Overlap your strokes slightly. Finish off by brushing a 3-inch brush loaded with paint lightly over the surface starting at the bottom and working your way up.
If using a 3-inch brush, work from the top down and paint in small, 10-inch high sections. Use vertical strokes. Once the entire door is painted, drag the empty or mostly empty brush up the full length of the door to smooth out the paint.
Allow the paint to dry thoroughly. Repeat Steps 7 through 9 to apply a second coat.