I presume that you mean a top coat on “vitreous” enamel and not hard
plastic. Because enamel is SiO2-based it is pretty “tough”, although
it can be scratched. I, for one do not use a top coat of “soft” or low
firing clear enamel (flux) except in the case of wanting to increase
the depth of the enamel to meet the metal surface without increasing
the saturation of the colour.
May I suggest, for starters, these books:
Practical Enamelling and Jewelry Work, Brian Newble, 1967 Enameling
Principles and Practice, Kenneth F. Bates, 1951 A Manual of Cloisonn�
and Champlev� Enameling, J.P & J.L. Strosahl and Coral (Barnhart)
Shaffer, 1996 Cloisonn� Enameling and Jewelry Making, Felicia Liban &
Louise Mitchell, 1980 Enamel Enameling Enamelists, Glenice Lesley
David in Victoria, British Columbia, happy because the garage, all
brightly painted and lit, is now a little studio.