Roy Noyes replies: I wonder what caused this question. Is it an actual or an imagined problem? I lived in a 200 year old house and had paneled exterior doors that were in excellent shape. They were painted on the outside but I am sure that they had no other special treatment. The only way, which I know of, to be sure of preventing the problem described is to put on an aluminum storm door. This will prevent liquid moisture from getting into the joints but will not prevent dimensional changes due to changes in humidity. Rot in this joint is not usually a problem.
Varnish and other clear finishes do not hold up well on exterior doors because the rays of ultra-violet light break the molecular chains in the finish and it peels, requiring refinishing every few years.
Painted doors are not affected by UV light but may have a problem if the panels are “glued” to the rails by paint. Then differential expansion, due to humidity or other moisture, between the long grain in the rails and the cross grain in the panels may cause the panels to crack. This is especially true if the panels are made from flat sawn boards. In many old houses, they minimized this problem by making the panels from quarter-sawn lumber, which has about one-half the dimensional width change of flat sawn lumber.