I finished a faux painted wood grain garage door today.  This door was brand new and white as white can be.  First thing I do is wash the door down really well and dry it off.  Then I prime the door and then paint the base coat paint on.  The first picture here shows after the base coat paint is done and the sun burst frames around the windows are painted in.

This next picture shows you how I tape off the individual panels for painting.  The tape is yellow.

Once the panels are done I tape off and paint the vertical pieces between them.

Then I tape off and paint the horizontal pieces.

Here's a couple of pictures of the finished door.