For the coating, have you investigated a two part colored epoxy resin
such as Ceramit. They have a clear version that is cured by time and
not heat. It is important with this type of resin to make sure you
are mixing in the exact proportions that are recommended by the
manufacturer, otherwise the product will not set up properly.
I think this is a place to start the investigation. Maybe someone
who has lots of experience can add their comments on using this type
of product for your application.
Paste Solder and Powder Solder for Metalsmiths and Jewelers