If you'd like to use what professional engravers use, you'll use Grobet USA's Background Antique and Aul-Solvent. It's simple and easy to use and it'll get you professional results. Simply brush on the Background Antique with a small artist's brush, let dry for 6 minutes. Then highlight bright areas by wiping gently with a lint-free cloth and Aul-Solvent. The pics below show the results and the product. Here's one place where the product is available:
https://www.esslinger.com/black-back...BoCGh4QAvD_BwE