The simple rule: Generally... the longer the finish takes to dry the more flexible and durable it will be. Quick drying finishes, six hours or less tend to produce a hard and brittle coating and the life expectancy of the product in an exterior application can be limited.
Why would you ask, is flexibility important? Doors by their very nature are abused; they are opened and closed, often banged or slammed. Wood is a flexible material that apart from the stress of opening and closing it is dimensionally altered by changes in temperature and humidity. If you live in the South and your door has a southern or western sun drenched exposure, it may experience a temperature change from 140° to 70° Fahrenheit from its outside surface to its inside surface every day. This causes tremendous dimensional stress on the wood fibers and the finish that is adhered to it. Under such circumstances a hard brittle finish will tend to crack at the edges around moldings and allow moisture to penetrate the surface and shorten its life expectancy.
So which finish do you choose? For most exterior doors that are not exposed to direct sunlight and are protected by a porch from rain and other environmental hazards, a six to 10 hour, medium-oil Polyurethane will provide an excellent finish. With a little care and maintenance such a door will provide its owners with several years of protection.
(Polyurethane does not hold up well to ultraviolet rays and the finish will break down after a few years of continued UV exposure.
For exterior doors that are challenged by adverse environmental conditions such as a southern or western exposure to direct sunlight and heavy UV rays, and no porch to protect it from rain and other harsh weather, an 18 to 24-hour cure medium-oil Spar Varnish is recommended. A Spar Varnish is more expensive and labor intensive to apply. It also takes longer to cure! But if properly applied and allowed to cure it will significantly outlast a Polyurethane finish under the same conditions. Restore Your Door uses premium quality Pratt and Lambert Vitelite Exterior UVA Spar Vanish that requires 24 hours between coats.
(Manufacturing field tests have established that Spar Varnish is substantially more resistant to UV rays and resistant to premature breakdown or crumbling.)
This source information was gathered from numerous articles written for the Taunton Press, Fine Woodworking and Woodworking Times. To further your knowledge on wood finishing we have provided links to these publications.