PET is often "subbed" by electron bombardment followed by a layer of gelatin with hardener. Or, PET is coated directly with the bombardment in a sequence called ILEB (In Line Electron Bombardment).
Other supports are often subbed by an organic polymer, a solvent and which is mixed with acetic acid which is then coated and evaporated evaporated. Gelatin is placed on top of this.
Ammonia and most other alkalis dissolve gelatin by denaturing or hydrolysing the amino acid links.