I am still learning to coat glass, but I have never seen Mark Osterman sub his plates nor put "edges" of material on his plates. I have seen him roughen the glass with a file or sand paper before use. He cleans any glass shards off of course. He explained that the rough edges give more edge adhesion.

Friends at Kodak that were involved in making plates said they did none of that, because plates were made in large sheets and cut apart before packing. They told me that chrome alum hardening enforced the adhesion and hardening both. I have seen that work myself. I changed to chrome alum and saw a big improvement in edge and overall adhesion.