Fish'n Jim wrote:
Not sure exactly what the plastic is, mostly tenite and PVC in that era.
Tenite is known for the white film degradation. See link below.
I was surprised the surface wasn't more disturbed requiring body work, if that occurred, so maybe something just contaminating. Must have been living a sedate garaged life. When they sit out in the sun with the windows up and the internal temp can reach the Tg of some of the plastics, they'll have no choice but to move/crack, etc..
I don't know what the mechanism of bending/distortion was but it's either from plastic shrinkage along residual molding stress lines or swelling/deformation of the door card and pushing/tensioning on the arm rest(creep). I doubt 'over' tightening would do much except crack the parts. Dimensional stability of early plastics was not that good/not scientific. '50s is the infancy of 'modern' plastics. Today, they model out the injection molding, custom formulate the material, etc. in the design of the parts where in those edisonian daze it was just make parts best guess and throw them out if they weren't right, aka trial by error.

Anyway - looks nice and good work all around.https://en.wikipedia.org/wiki/Tenite[/quote]

Good information, thank you!

Thoughts.....
-I suspect my car was garaged most, if not all of it's life, as fading is minimal for a 71 year old car. We don't get lots of humidity here on the prairies, but I have no idea how it was stored in the years before I bought the car. It could have been in an uninsulated shipping container. There is surface rust on unpainted surfaces such as the back side of the window trim, so I suspect it saw some high humidity at some point in it's life.
-The surface of the plastic is really not degraded at all, so maybe as the Wiki article suggests, it's a surface mold, and not the plastic itself breaking down. Maybe keeping the car clean and dry will prevent it from coming back?
-Without a doubt, the distortion is directly related to the force applied by the screws that fastens the armrest to the door. I was able to warm these areas and remove most of the distortion by applying force in the opposite direction.
-Regardless of how the car was stored, and how little they knew about plastics in the 50's, that these armrests survived as well as they did is impressive.