I've been working this way a few times, manually (have no plotter yet) using
some kind of dentist tool, nearly 6000rpm, round-tipped cutter.
Just coat the board with thin layer of lacquer and "draw" all that you need
with light strokes of the cutter.
It will make nice, sharp edges, width of insulation path depends on cutter.
If you want only to scratch the lacquer with sharp-tipped tool, the
insulation paths will be too narrow and the lacquer can crack near the
edges. It will never happen if using rotating tool.
So the idea is to use fast-spinning tool with round tip, you need only to
push it lightly against the board, so it can be done very quickly and
easily.
The other technique I have heard of is similar, only the traces are "milled"
when there must be copper not where insulation will be. After that the board
must be tin-plated. Then wash away the lacquer and etch the board in some
kind of solution. Just I don't remember which one of the most popular
etchant solutions doesn't dissolve tin. But it wasn't anything very special.
KPL
