Nice! I tried doing this a few years ago with a RepRap build. It didn't go well! Bad marker, pigment that dissolved, not enough ink layers, but the most interesting one was with Staedtler markers which sorta dissolve previously laid down ink when I made multiple passes. Ended up doing toner transfer after giving up.
I wonder how would a thin plastic film cut with one of those vinyl sticker cutters would fare for masking.
Oh huh, I'm using an Edding 140S, in case it matters. It was what they gave me when I asked for a permanent marker, and it worked beautifully. Didn't dissolve at all in the etchant, yet wipes right off with some alcohol.
As long as it works or can be fixed with some bodge, I'm okay with that for prototypes. I do tend to tin the traces wherever they seem 'porous'. For broken traces, I use wrapping wire core soldered on tracks.
I wonder how would a thin plastic film cut with one of those vinyl sticker cutters would fare for masking.