Ah...6 panel doors can be a real pain in the butt. Seams like you are forever dipping your brush in the paint can.
By the the time you get halfway finished,boredom sets in and you are ready to call it quits.
Here is a technique I have used many times and works really well.
Use a good brush (usually around $13)The cheap brushes tend to streak and drag a lot. A 2" angled sash brush works really great.
I usually paint a third of the door at a time.
Apply paint to the door using a 9" roller. Coat the panel and other flat surfaces and smear some into the contoured areas.
The purpose of this is to get the paint on the door.
Next, use your brush to move the paint around.
Do the contoured areas. Then do the panel and finish up with the other flat areas.
Always stroke your brush in the direction of the grain.
If you are painting on a hot day, you may have to paint smaller sections at a time.
Once you get the hang of this, you paint a 6 panel door in about 10 minutes
Good luck and Thanks
Be A Paint Pro