Many amateur do-it-yourselfers have difficulty painting doors without leaving unattractive brush strokes in the final finish coat. Combat this by applying the paint with a roller. Unfortunately, you cannot use a roller alone, or you will end up with bubbles and roller nap marks. Fortunately, there is a way to create a professional-looking finish with a roller. However, you must use the roller in conjunction with a brush. In addition, you may need to dilute the paint before application, depending on the environment you are working in.
Open the door. Remove the hardware with a screwdriver.
Abrade fibreglass and previously painted doors to promote adhesion by sanding them with 220-grit sandpaper. Skip this step if the door is bare wood or metal.
Slide masking paper beneath the door. Tape the edges down with a low-tack blue painter's tape.
Pour one gallon of primer into the five-gallon bucket. Apply a coat of acrylic latex primer to the door, using the roller. Smooth the primer before it begins to dry, using a four-inch paintbrush, manufactured for use with latex paints. Use a galvanised metal etching primer if the door is made of metal. Wait two hours for the primed door to dry.
Clean your painting tools with water.
Coat the door with acrylic latex paint. Apply the paint just as you did the primer. Use a clean, new nap roller cover.
Because doors are relatively spacious surfaces, the first several brushstrokes tend to dry at different rates than the latter ones. This creates an uneven-looking finish. By using a roller, you are able to coat the door with paint quickly, allowing you to smooth the finish with a paintbrush before it begins to dry. You do not need to prime previously painted doors. If you are working in high temperatures, dilute the paint to promote a smoother finish by adding one gallon of water to each gallon of paint.
Do not paint over an unprimed bare metal or wood door, or the finish will fail. Do not use a dropcloth instead of masking paper, as dust sheets tend to bunch up and stick to wet door paint.