As brass instruments get dirtier they will typically start having minor to major valve issues and tuning slides may get stuck. Dirt may cause slide issues for trombones. Green sludge and other grime may also start to build up. Tubes may start rotting out and an instrument may need costly repairs. Cleaning your instrument regularly can help prevent build up.
Nothing can interfere with an instrumentalist worse than sticking valves or a dirty trombone slide. Many times the only issue with the valves or hand slide is dirt and oxidation and a good cleaning will get things moving again. Dents or other damage to valves and hand slides would require additional work to get the instrument in playing condition.
If a tuning slide is not greased frequently, the slide can get stuck. If the main tuning slide is stuck the tuning of an instrument cannot be changed. Playing on a horn with a stuck tuning slide will only increase the corrosion. And submerging an instrument with stuck slides in water will make the problem worse. While it is rare, occasionally horns with stuck slides that are played frequently over a long period of time might be impossible to be loosen. At this point, tubes would have to be replaced.
Red rot is a term commonly used to refer to spots on a brass instrument where the zinc has been chemically eaten out of the alloy by the acids from a player’s saliva or physical contact. This leaves behind a spot of brittle copper – hence the name red rot. If an instrument goes too long without being cleaned, and depending on the individual player’s acidity levels, red spots aka red rot, appear and if the red rot gets serious enough the instrument will need to be patched or the tube replaced.
Cleaning a horn at home can help, but typically household chemicals are either not strong enough to remove copper oxidation that builds up or so strong that they can damage the instrument. Getting a professional chemical clean once or twice a year is recommended to prolong the life of your brass instrument.