Painting flat doors and achieving a quality smooth finish can be difficult but with a little practice and the correct approach a high quality finish is achievable.
Trade painters use a technique referred to as crows footing when painting flat doors to achieve a smooth level and even finish. This technique involves loading a paint brush and then using a series of rotating crosses to evenly spread the paint which is then tipped off in vertical movements with the tip of the paint brush pressed lightly on the door surface. This method is not the fastest method and crows footing technique can be difficult for inexperienced painters to master.
Another method that is much faster is to use a paint roller which enables almost anyone to achieve an even film build of paint over the entire flat door face. A draw back with finishing doors using a paint roller is the paint roller texture can remain and high gloss levels accentuate this paint roller texture.
An excellent method of overcoming paint roller texture on a door is to use a fine paint brush to tip off or layoff after the even film build of paint has been applied with the paint roller. This method involves resting the paint brush lightly against the door at the top and let it fall while resting against the surface to almost the bottom of the door. Continue from one edge of the door to the other and then repeat from the bottom of the door backup to complete "laying off" the entire door face. This method is fast and helps achieve a very even film build and result in a very fine finish. If done correctly it should be difficult to tell that the door has been either paint rolled or even painted with a brush.
Of course paint the door edges and cutting around door handles and hinges before commencing to roll the entire door face and subsequently laying off with a paint brush.
An alternative is to spray paint the doors but this requires that the doors be painted prior to anything else and then cutting back into the doors and frames or masking around the doors and frames completely or even removing the doors for spray painting in a separate location. A real benefit in spray painting doors and frames could be that much harder wearing higher course level paints such as two pack polyurethanes could be used instead off softer water-based acrylic enamels or oil based alkyd enamels which tend to yellow badly these days by comparison.