Kastus is sprayed onto the surface of the glass as a fine mist just before it enters the tempering stage. Once this liquid is heated it crystallises and sinters (embeds) into the surface of the glass. This locks it into place and it becomes a permanent part of the glass surface. It isn't like a normal coating that just sits on the surface. Being embedded means it's extremely durable and will last for the lifetime of the glass. It never has to be reapplied. It fits seamlessly into existing manufacturing processes.
Kastus is embedded into the surface of the glass during manufacturing.