http://youtu.be/J6ydw23TCD8

To avoid “lap marks” while painting a ceiling, keep a wet edge across the entire ceiling from corner to corner. A wet edge simply means to paint from one side continuously across to the other side and to do it all in one shot without letting it dry. If you paint half way, let it dry, then come back to finish, you’ll end up with a “lap mark” on the edge of the first section that dried.