Rebonjour Roger and all,
1/ the typical failure is hairline cracking résulting in entry of air, which then ionizes, eventually causing a,carbon track along the crack surface.
Another failure mechanism is entrapment of air bubbles due to lack of vacuum, or insuffisant time, temp, annealing.
2/ The plotting material and process was propriétaire and cost perhaps $50 eack in volunteer production, minimum was 100-1000 per run!
3/ My HV potting work was 1980s...1990s. Even if I knew what the material was, I have forgotten and lost the old documents.
Nonetheless I shell search my archives and pass on any tips.