I’ve been reading Fred Ball’s book, Experimental Techniques in Enameling about using liquid enamels with a white overlay on sgraffito.
Sgraffito is a type of decoration made by scratching through a surface to reveal a lower layer of contrasting color, typically done in plaster or stucco on walls, or in slip on ceramics before firing. Sgraffito and Sgraffiti derive from the Italian word graffiare ("to scratch"), ultimately from the Greek γράφειν (gráphein, "to write").(https://en.m.wikipedia.org/wiki/Sgraffito).
When using the sgraffito technique in enameling you can sift or paint enamel onto metal and scratch through it or you can enamel a color on first and then sift or paint on top of that and scratch through to the color layer. Some of the tools you can use for scratching are toothpicks, needles, xacto knives, comb, toothbrush, Q-tip, and chain.
I generally paint a low-firing liquid white onto bare copper, let it dry and then scratch through it with one of the above mentioned tools and fire it. Then I sift several thin layers of a ground soft white on top, torch-firing each layer separately, usually overfiring. All of the enamel pieces here are fired only with white enamels.