I asked this same question in the musical instruments repair and
builders groups, as I had a set of spun-brass ship horns that needed
just such a treatment. I was told that the lacquering of brass
instruments is something of an art form, with many of the amateur
instrument builders frustrated by trying to get a perfect, drip-free,
sag-free glossy clear finish. I've had the best luck with gloss Krylon
"Lacquer" and no luck with the Krylon or Rust-oleum clear acrylic
enamels (they have always given me a slightly hazy finish that kills
the sharp specular highlights of the bare polished brass).
I don't think there are any lacquer dipping processes used, since that
would be sag city, but I do suspect that a lot of the cheaper
instruments, and perhaps even some good ones, are using a clear
powdercoat now. Door knobs, handles, locks, lamps and other polished
brass items are certainly powdercoated nowadays. Sears even sells a DIY
clear powdercoating system, as to the major industrial supply places
(Grainer, MSC, McM, etc).
If you do lacquer, it won't last forever. Outdoors it might not last
long at all. Amongst boat owners, there are two schools of thought
regarding exterior polished brass adornments: polish weekly versus
lacquer and strip yearly. For my brass truck horns (from a tugboat), I
chose the latter, and so far they've held a good coating outdoors for 6
months. By this time next year, I expect to have to strip them (acetone
bath) and repolish, relacquer.