How to... paint a door
If you’re painting a door, the easiest thing to do is take it off its hinges so you can get to the edges easily (this also enables you to paint the inside of the frame).
Every time you paint where an edge meets the front or back of the door (depending on which side you’re doing), run a cloth or your fingers along it to make sure that a ridge of paint doesn’t form where you can’t see.
This will ensure that you get a good finish and won’t have problems refitting the door because of ridges of paint.