There are lots of useful tips about how to paint a panel door that can make the job straight forward with a happy outcome. Before you start, make sure you have the following items: wood primer; 2 or 3 paint brushes - 1 inch, 2 inch and an optional 3 inch brush; suitable undercoat; suitable topcoat; fine sand paper; white spirit and cloth; and a screwdriver.
The first step is to remove the handles, plates and locks to make the painting easier. Dust behind thoroughly and blow out dust from the lock. Check for any flaking paint, and remove with a scraper. Feather the surrounding paintwork with fine sand paper to even the surface, fill open joints or cracks with wood filler and rub down. Wash the door down with sugar soap and allow to dry.
Before painting, make sure you wedge the door open with two flat sticks or a rolled up newspaper. Put cardboard or newspapers under the door to protect the floor or carpet. Assuming your paintwork is in sound condition, you may not need primer or undercoat, although you should sand down existing paintwork. When applying the top coat there is a correct alphabetical sequence for painting the panels and edges which will give the best results. If you have an outward opening door that can be seen from the stairs, first paint the top edge, and check for paint runs. Next paint the hinge edges on the doors opening into a room or outwards to the exterior. Paint the beading around the panels with the small brush ensuring you catch any runs. Coat the panels themselves, with smooth strokes of the brush following the grain of the wood. Next paint the vertical stiles with smooth vertical strokes following the grain of the wood. Then coat the horizontal rails using horizontal strokes of the brush. Next, coat the vertical runs using smooth vertical strokes, ensuring you leave an even finish where the joins of the horizontal rails meet. Finally coat the edges of the door with smooth vertical strokes ensuring to catch any runs or build ups of paint on the corners.
Allow the gloss to dry before rubbing down with a fine sandpaper, wipe door down with a clean cloth to remove any dust, and apply a second coat in the same way.
Four main tips:
Keep checking for runs in gloss paint
Most likely places for runs are bottom corners of panels and across the top of panels
Check for a build up of paint in the corners of panels by dabbing with the tips of the brush bristles, using a gentle dart throwing action.
Always try and remove runs by using an upward motion of the brush but where there is grain, leave off in the direction of the grain.