In building a good foundation get to know your vitreous enamel flux. 

Like any painting technique there are various primers for various jobs. In art of enameling we have quite a few fluxes but they produce various findings. You have heard of a car having the rust primer coat = flux, before the color coat, If you ever finish a beautiful wooden object you would have the sand and sealer = flux, before the varnish coat. And preparing metal to enamel is the same way. See how I have prepared an enamel flux test plate.

Enamels are reactive to metal, not all but 90% I would say. So flux coats are usually necessary, and there are several hardness of these fluxes from available manufactures.

Next, I know what does that matter? visit my thoughts, understand vitreous enamel flux, with my enamel flux test plates