Once I saw a TV report from the Rolls Royce factory where fine gold
lines were painted by an enormously skilled artisan who used a brush
with very fine hair.  The special thing was, that although the brush was
quite narrow (1/8 inch range), the hairs were two or three inches long;
it looked more like a whip.  The hairs trailed along the lines for a
long distance to smooth out any irregularities in paint deposition.
That is a method that works with straight lines on a flat surface.

But the fact that the lines to gild are engraved or etched suggest that
the technique to use is the same as the one practiced by copper-plate
engraving printers.  That is, you fill up the lines coarsely with the
gold paint, perhaps using a plain brush, and immediately afterwards you
wipe off the surplus paint that is on the flat surface beside the
engravings: first coarsely with paper or cloth, but finally with the
inside of your hand - the pad formed by muscle and skin below your
thumb is an irreplaceable tool.

A prerequisite is of course that the surface (not to be gilded) is flat
and well polished and that it is not softened by the binding agent of
the gold paint.  Most probably you will also have to enhance (deepen)
the lines before gilding using an engraving tool.

Johan Liljencrants