After years of use unpainted French doors usually need to be cleaned up; not just from dirt and dust but standard wear and tear. If you think your French door needs a paint job doing it is an easy task for the DIYer.
Here are some suggestions on how to go forward and improve the quality of your home and your French door.
What you will need: primer, good quality paint, a 4 inch wide brush and a half an inch thin brush, painters tape, a ladder, drop cloth, spray bottle with soap and water and a rag.
Put down a drop cloth and lightly wash the door using a spray bottle with soap and water and a rag.
Let the door dry.
Remove the door from the wall and take off any hinges.
Place the door on anything that will hold it above the ground. Wooden horses would be ideal as they would also provide an ample height from which to work. Make sure whatever you use does not mark or make impressions in the door.
Protect the floor where you will be working with plastic or a drop cloth.
Mildly sand the wood with sand paper and wipe down with a mildly damp rag to remove any lose particles
Apply primer to one side and let dry. Then apply a second coat of primer and let dry completely. Do the same for the other side.
Don't worry about paint getting on the glass since it will come off easily with a razor.
Apply the paint in the same manner as the primer: one side first, let dry, add a second coat, and then flip the door over and do the other side.
Reattach the door to the wall.
Using a razor blade scrape off any paint that got brushed on the panes of glass.