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I understand "red rot" is a gradual change in brass due to a "fade out effect" that lets zinc disappear, so the copper is left behind. This effect will affect (He,He) the whole volume of metal and leads to an actual change in material, eventually. If you polish these spots, you´ll get nice and shiny areas of copper, not brass.
"Patina", on the other hand, is just a corrosion process that takes place on the surface. Once a layer of patina has built up, it´ll prevent further corrosion in the metal underneath. It´ll come off with a good polishing session to produce a nice, shiny brass surface.

I own this old (maybe 30 years) Getzen Eterna flugelhorn. It has been out of use for maybe ten years now.
It does have quite a few scratches, but operates and sounds just fine. I´m thinking about selling it to a bandmate so it can serve its original purpose again.
I want to make sure I´m not selling junk to a friend, so here´s my question:

All those places where lacquer has been scratched or has come off otherwise are of bright brownish-red colour, where they (the scratches) used to be brass-coloured in the past. The surface is smooth, though, so it´s not like it has become "rusty" (yeah, I know, brass doesn´t rust) to the point that material actually is wearing off, and I´m not observing any tendency of holes building up, which would be the case with old leadpipes suffering from "red rot", AFAIK.

I figure this color is due to some kind of corrosion process and would like to ask the repair-people on this board whether this is likely to get any worse. I haven´t tried yet if the red stuff will come off with polishing.

tubeast wrote:I figure this color is due to some kind of corrosion process and would like to ask the repair-people on this board whether this is likely to get any worse. I haven´t tried yet if the red stuff will come off with polishing.

It's not uncommon to see a little surface dezincification on an old instrument, where the patina has a reddish tinge.

That is not red rot. Red rot is not a surface issue, but rather a perforation issue. It happens from the inside out.

You'll see red rot first as a small dot of crusty calcium. This a spot where acidic moisture on the inside of the instrument has tunneled through to the outside, by attaching the zinc and leaving spongy porous copper in its wake. It will be a pinhole and then the pinholes will become more frequent. Eventually, the metal will be easily crushed in the affected spot, as the dots start to connect.

The process is similar to rust-through on the body of a car. If you scratch the paint down to bare metal, the surface will rust and leave a red stain. That is initial cosmetic. But if the metal is allowed to get and stay wet on the backside (or inside the fender or door panel), it will rust until the rust perforates the metal and appears as bubbling paint on the outside.

Rick "who watches for pinholes on his instruments" Denney

Cool, that pretty much answers my question.
I´ll go ahead and let my pal make an offer, then.