We see a lot of (French) horns come in with stuck rotors that are otherwise pretty clean. Despair not, horn players, that's the last mention you'll see here of that word referring to things from the upper half of the Iberian peninsula. I have to provide terms the layman can understand, you know. Incidentally, if you are a layman reading this, you've come to the wrong place. Tumblr is a few pages over.
Often these are instruments that get regularly used, but one day one or several of the rotors just starts sticking. Sometimes it's because there's buildup on the rotor body and the casing, which just happens naturally over time. It can also happen if the rotor isn't getting oiled regularly. Other brass players seem to have no problem with regularly lubricating the moving parts of their instruments, but a lot of the horn players I see seem to think that the instrument gets oiled when it's serviced and then doesn't require further oiling. Perhaps because the rotor is sealed up and so hard to get to, they figure the oil stays in the casing. In fact, the rotors need pretty regular oiling. Even though the spindle oil is pretty thick and needs less frequent replenishment, the rotor body is the part that's being constantly subjected to water and whatever organic matter is coming from the player's mouth through the leadpipe and into the rotor block. Because of that, the rotor body can benefit significantly from a protective layer of valve oil. We often think of valve oil just being for lubrication of valves, but it has a few other functions, one of which is acting as a protective film on the face of the valve that repels water and other materials. While valve oil doesn't act as a lubricant in rotors (all the bearing surfaces requiring lubrication are on the spindles and should have spindle oil), a few drops down the slide tubes about once a week will help keep the rotors clean.
Valve oil provides another benefit as I mentioned, which is repelling water. I believe that water in the casings is the other big cause of sticky rotors. Often when I remove a sticky rotor to clean it I find that the rotor itself is pretty clean, but a huge amount of (gross) water will come pouring out of the casing and be beaded up all over the face of the valve. Water is like anti-lubricant. It has relatively strong cohesion and adhesion properties, and droplets have pretty thick surfaces. When water gets between the rotor body and the casing wall, it wants to stick to all the surfaces and itself, creating a vapor lock that stops the valve from being able to move alongside the casing. A lot of times, just getting the water out of the casings and slathering the rotor bodies with valve oil is enough to get things working, but that's beyond the capabilities of a lot of players who aren't comfortable with removing their rotors and reassembling them.
For those players, though, there may be another way. A horn player who regularly patronizes our shop is an engineer and was on board with my idea about vapor lock, so he figured that if he could remove the water from his horn he could remedy the sticking rotors. His first idea was to use a shop vac set to "blow" to send air down the bell and through the rotor block, back out through the leadpipe. After agreeing that that could send some shop vac contents and other dust into the instrument, he instead tried sucking the water out through the bell. By placing the end of the shop vac hose into the bell to create a tight seal, he was able to drain a good bit of water from the rotor block, and saw an instant improvement in the performance of his rotors. A week later, things were still working great.
There's still no substitute for regularly oiling your rotors. But water buildup is a problem that can creep up on any instrument, and I think the shop vac method of evacuating water can be a good way to maintain your instrument between professional cleanings, which should be every 1-3 years, or every time you feel like sending your repair tech some money, which is probably pretty frequently.
