Painting houses you will certainly encounter a lot of doors!
When painting a door it is important paint systematically in order to hide your brush strokes. Doors are a focal point as they obviously designate entries and people are always looking for or at them. Compared to some walls in houses, a door is rarely stuffed away in the corner and hidden from your sight. Doors are in plain view. One of the most obvious flaws on a door is brush marks. Roller marks can be troublesome too. The goal when painting a door is to use your brush first then roll with a foam roller over the larger open areas covering all the brush marks.
In this example the top of the door is being painted here. First you paint the “wells”, the area in between the square areas. Then you roll the small square and then the areas around the square’s. When you roll you will cover up the brush marks that are outside of the “wells” around the square’s. The opposite way would be to roll first then brush. If you would choose to do this then your last work you would see so there would be brush marks around those square areas.
Another example is around the door handle. You would want to brush around the handle then go as close as you could with your roller. Often people also use a brush only with doors and that is very acceptable. That is reserved for a skilled painter he would still want to stick to a plan to hide the strokes or make the brush strokes consistent. Here he would use his brush to go with the grain or length of the door panel.
Painting a door can be done quickly and with practice you will become a skilled technician.