If it is traditional enamel, fluoric acid. Care is required, the acid is poisonous. If epoxy based, then industrial epoxy solvent.
From what I learned about Hydrofluoric Acid in chemistry is that this
is one substance not to be messed with. To say care is required might
be a understatement for a substance that cannot even be stored in a
glass container. Among the many miseries of exposure to this acid is
permanent nerve damage. I’d rather make a new piece entirely than try
to salvage one with this method. Hydrofluoric Acid has a mean,
vicious, and long, very long lasting bite.
Small Scale Metalsmith