Why do you need a hard flux? Your flux is laid down first on the metal of your choice. You want a hard flux which means it will not become fluid as quickly as you color layers of enamels that are applies later. This is specially important when using warm colors.
In this pendant of cloisonne, it will be a deep layering of colors. I need an enamel base flux that will hold up for many firings. Bovano 3# is a hardest firing leaded flux I have found for silver. This will protect my warm colors from mingling with the base metal and turning the enamels the color of mud. FYI this flux and many other brand fluxes art harder firing than all your colored enamels. So it is not mean for a top coat.
If you are looking for a top coat to, let’s say to catch up one cell filling with the rest of your cells, use the lightest color of the family of enameling you are using! Or if you are happy with the depth of color you have achieved but the cell is not filled to the top of your cloisonné wires use the very lightest color of that family.
I recommend Bovano’s #3 or Ninomiya’s 110 the two best enameling fluxes i have tested.