Reddit mentions of Grafix 4-1/2-Ounce Incredible White Mask Liquid Frisket (WM4)

Posting from my phone but I hope this works:

https://www.amazon.com/dp/B00139SZU4/ref=cm_sw_r_awd_WkKevb16HT8MW

Three approaches I'd take:

1. Cut a mask out of adhesive backed vinyl on a knife cutter, then chemically etch or sandblast the positive.
   
   
2. Cut a mask out of chipboard or plywood on the laser cutter, and similar to above, sandblast the positive
   
   
3. Do a light (.003 inch) engrave using a CNC.
   
   If you scribe by hand, something like liquid frisket might work better than shellac or a bituminous material. Frisket can be painted to a fine resolution with a pure sable hair brush, which is what I do for watercolors (using a size zero for detail work)...never used it for etching, however.