Hard Enamel can be utilized on all coins when 2D surfaces are used.
3D coins must use epoxy as a protectant for painted areas.
Shiny metal finishes can have a stoning process done on the coin, which creates flat paint. Antique metal finishes does not allow for the stoning process. Hard Enamel or Epola replaced the traditional Cloisonné style.
Here is an example of Cloisonné. Hard enamel replicates this look.