And the fact that 1/4 inch plate glass was installed much condensation developed which dripped down to the lower rail attacking the wood from the interior side as well. (the down side of using non-insullated glass)
When the exterior finish failed-9 to 12 months for a Spar varnish. Linseed oil or tung oil almost immediately. And in some cases no finish at all.
(See Wingspread specs.) When it was discovered that the finish failed it may have been months before the weather permitted re- application.
This penetration of water eventually takes its toll, mold develops and rots the wood along with the mortise and tennon joints. Soon the doors can not be operated by a single individual for the bottom rails start their steady decline and soon can no longer seal into the bottom threshold properly.
Note: this condition was found with all usonian woods Wright used. Tidewater Cypress, Red Phillipine Mahogany (Red Meranti) and Redwood.
Doors that do not open to terraces and gardens close one in.They prevent experiencing the freedom of movement Mr. Wright originally intended. And in warmer months prevent the much desired flow of fresh and cool air. It is possible that it was at this time many original clients turned to air conditioning their homes, for lack of proper fenestration.
These doors must operate to fully experience usonian living.There are two ways to address the problem. A total rebuilt door or a technique/process that retains a large percentage or existing wood. One must cut away the lower stiles and bottom rail. Using a Dutchman joint replacing old rotted areas with a new bottom stiles with mortise and tennon joinery.
With todays building sealants and excellent water proof exterior glues on the market one can restore to "better than new" condition. And of course for the oil finish, the finish of choice is Sikkens.
Perhaps in the future I could post images showing the techniques I used to restore eight doors and two mitered windows on the "Dobkins" residence
designed in 1953,built in 54 Canton,Ohio.