Here are some tips for painting your home's exterior doors.
Wood exterior doors should be painted on the same schedule as the home's exterior body and trim - about every 4 - 6 years.
Stained exterior doors should be sealed with a varnish or polyurethane. You can add a coat every 2 - 3 years after a light sanding with 220-grit sandpaper. Stained exterior doors are like hardwood floors in that if you wait till bare wood is showing through, you've waited too long, and someone will have to sand the entire door down to bare wood and start the stain/finish process all over again.
Aluminum, vinyl-clad wood and solid vinyl doors do not need to be painted. Steel and fiberglass doors come with a primer coat, but they need to be finished with at least 2 coats of good quality latex paint - front, back and all edges.
Most wood doors are now made of engineered wood and are less susceptible to warping. If you're replacing an exterior door, and you just gotta have wood, make sure it is of the engineered variety. I prefer fiberglass doors as they seem to weather our high-altitude Colorado sun better than wood and definitely have higher R value