That is oxidation under the lacquer. It could be caused by little breaks in the lacquer allowing air to tarnish the brass. If you want it "purdy" then you would need to have it over-hauled or keep up on it with polishing... That would eventually wear away the lacquer.
Bill Vance
Martin-King 6/4 custom 4V BBb Tuba
Martin "Mammoth" 3V Sousa '27
Martin "Mammoth" 4V Sousa '29
Mirafone 186 BBb (being Oberlohed in Seattle)
I've seen this on almost-new horns. Apparently something was splattered on the horn before the lacquer was applied to cause extreme thin-ness of the lacquer or loss of adhesion.
Like another poster said.... about all you can do is either strip it or allow it to just 'wear away'.
I had a year-old horn in here a few days ago that had red-ish spots on the inner surfaces of the large tubes. There was no lacquer at all on those spots. It wasn't 'rot'.... just lack of lacquer.
Dan Schultz
"The Village Tinker" http://www.thevillagetinker.com" target="_blank
Current 'stable'... Rudolf Meinl 5/4, Marzan (by Willson) euph, King 2341, Alphorn, and other strange stuff.
Looks like it's also on the slides. That would be a lot of work to spot-buff with a Dremmel or similar tool, but if you need a project . . . . .
Try buffing down to the oxidation, cleaning with a laquer thinner, and then spraying with an aerosol laquer.
Dean E
[S]tudy politics and war, that our sons may have liberty to study mathematics and philosophy. Our sons ought to study mathematics and philosophy . . . in order to give their children a right to study painting, poetry [and] music. . . . John Adams (1780)
The horn is my Bach 869 "Mercedes." From the front, the horn still looks nice and shiny, the back side, for whatever reason, is the side with most of the scratches and "lack of lacquer."
Jordan
King 2341 with a Holton "Monster" Eb bell
Eb Frankentuba
Martin Medium Eb Helicon
If at first you don't succeed, skydiving's probably not for you.