Dunno if this has already been said, but my experience from painting
some bits (most notably my stem - see
http://www.atnf.csiro.au/people/Suzy.Jackson/frame/audax_stem_painted.jp
g) which have un-painted lugs, the best way to get a good line around
the lugs (in my case with the same colour as that on the rest of the
tubes, rather than a contrasting one) is to use a "ruling pen", rather
than a brush, which is like an old-fashioned ink-pot style pen. These
make nice, consistent thickness lines, and would make lug lining
dead-easy.