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.