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/TeniteThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=267165