TromboneChat

Has anyone seen/done things like this before?

https://www.ebay.com/itm/Bach-stradivar ... SwrTFdWYru

I've seen it on some Shagerl and BAC horns, and a few for sale by James Burton iii, but I don't know how they go about doing it. It's a super cool look, are there any major drawbacks to doing something like this to a horn, or is able to be polished out?

Here are some examples: https://www.youtube.com/results?search_ ... rass+brown . I would expect that it might take some work to polish out. I'd experiment on spare brass parts, tubing, scrap, etc.

Any patina that is induced chemically, is only on the surface of the metal, and it can be buffed off if desired. There are dozens of different finish possibilities, based on the chemical used, and the color derived from that chemical. Do some Googleing for more info. The instrument would be lacquered to seal the finish, and prevent it from wearing off or changing.

Woodworkers sometimes use an artificial brass ager for antique restorations, check this link.

https://www.rockler.com/darkening-solution

That... looks like crap, sorry. All they did was take off the lacquer and use brass black, or something similar. They probably didn't properly soak it in water for 24 hours to remove the oxidizing agent either.