Pure Gum Spirits or Turpentine is needed for cleaning and thinning varnish, paint, and enamel when refinishing or staining your exterior doors.
Do not be stingy with turpentine; it is an excellent wood-conditioner, especially for solid wood doors. Always have a rag soaked in turpentine lying around to clean up after sanding the wood doors, painting your exterior doors, or to get crud off your hands; or if you are me, your clothes.
Salvage your turpentine by pouring what you have used for cleaning brushes into a container (pickle jars work) and sealing it tight. In a few days after you finish or refinish your wood doors, you will be able to drain off relatively clear turpentine for future finishing excursions.
If you are working all the time with oil-based paints, keep a jar on hand full of spirits and soak your brushes to keep them conditioned.