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.
J Collier
Small Scale Metalsmith
To say care is required might be a understatement for a substance
that cannot even be stored in a glass container.
I’m usually rather impatient with posts warning intelligent adults
about the dangers of workshop chemicals, but I’ll make an exception
for HF. This stuff is not only dangerous but sneaky. If you contact
it, you may not even know until hours later, when it’s too late. HF
burns may not be visible or painful, even while the HF is migrating
to your bones and turning them into calcium fluoride. Specialized
medications are needed even for first aid treatment. Read the linked
MSDS.
http://www.bu.edu/es/labsafety/ESMSDSs/MSHydFluoricAcid.html
Al Balmer (who once spent half a day in the company clinic
undergoing various unpleasant treatments because a few drops might
have splashed on him.)
Sun City, AZ