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I just bought an economical little used Mack Brass 4/4 F tuba. Plays wonderfully but has this bell rot pictured. Most of the posts on this board deal with red-rot. I don't believe that's what this is. Two repair techs have told me this is flux corrosion and would just come back if we buff it out. Has anyone here buffed out corrosion before? Can you share techniques? I would think a very light touch with a fine wire dremel tool would be a good start, cloth Buffington wheel with rouge and a little spray laquer would be a vast improvement. So what if it comes back? Do it all over again!
Comments appreciated.

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A dremel buffing attachment would work well.

I know there's a lot of people who are against brasso but I think it doesn't remove enough metal to scare you if you use it once or twice.

High grit sandpaper and some oil also works.