Liberty Polish. Does exactly what your asking for - works just like
Tarn-X for brass. It's non-abrasive with a very slight ammonia smell.
It's an absolutely must-have if you own/make anything of uncoated brass.
I occasionally turn solid brass candle holders as gifts. I leave them
uncoated, but include a small glass jar of the stuff in the box.

Here's where I get it:
http://www.rejuvenation.com/relatedproducts/polish.html
(no affiliation, it's just close to my house, the guy at the counter
suggested it, and it worked the way he said)

- Bruce

"Doc" wrote in
ink.net:

Is there anything that comes in a brush-on or at least spread-on form
that will take spots of oxidization/red rot crud off brass without
eating the brass itself and does most of the work for you, sort the way
Tarn-X works on silver? I'm giving an old trumpet a going over and want
to reduce some of the tedium in resurrecting the sheen of the metal
before lacquering it.

Also, what sort of surface preparation is recommended before lacquering?
Please no "take it to a repair shop" answers, the whole point is to be a
DIY project.

Thanks for all shared wisdom.