Pictured above is an all too common sight on modern horns: Brown discoloration beneath the lacquer that shows up after a while. I see this on low quality horns as well as the name brand pro level horns. My own Bach Stradivarius trumpet had this issue around the bell wire. This is what happens when you speed up production and don’t properly prep a horn before lacquering. Soldering flux is very acidic and if you don’t completely wash it off and neutralize it after soldering, you get discoloration of the metal and the finish. This can happen with repairs and it happens at the factory when the instrument is built. In this case, after the neck was soldered together with the neck tenon it wasn’t properly cleaned and they lacquered over the metal while there was still soldering acid on the joint. At first, the horn looked fine but after a while the acid reacted with the lacquer and produced this gross brown stuff. You can see how it’s spread out from the joint where the acid was concentrated at. This is easily preventable with proper cleaning and neutralization of the part before it is lacquered. Unfortunately, you can’t repair this issue without removing the original lacquer from the area and re-lacquering it, which in most cases isn’t worth doing. So you’re stuck with this ugly mess.