I have a couple of questions about finishes.
First, Much has been written about silverplate and how to maintain, but what about lacquer? What kind of cloth or polish should be used? Any particular brand or a specific type?
Second, Can silverplate or lacquer with worn through areas be repaired and look good or does the whole horn have to be replated or re-lacquered for a proper repair?
1. Lacquer can be cleaned using a lacquer polish and a soft cloth. Pledge (furniture polish) also works well.
2. For Aesthetic repairs lacquer can be touched up. Sometimes it can be done where visually it doesn't show, though sometimes, practicality of it will show a bit of overlapping between the original lacquer and the touch up where the colour may look a bit different.
For Silver, most spot plating methods are simply too thin and don't hold up so touch ups are less common unless if the whole part can be unsoldered to send off to be replated. The bare points can also be polished up and touched up with lacquer to seal them again.
Silver does provide a better resistant finish to wear than lacquer. Hand grips cab be an option to prevent/cover wear in hand contact positions as well.
Looking good is a bit subjective to the eye of the beholder but for the most part, full refinishes in lacquer or silver plate are rare (except in custom restoration work like Dan Oberloh) due to the cost and availability of electro plating or facilities to do proper full relacquering.
What about other finishes, such as nickel? (said the owner of a new nickel-plated Schiller)
Nickel is a harder plating than silver. It won't tarnish black, but will get cloudy over time. There are some nickel polishes available, abrasive and non abrasive. Honestly, when they do tarnish, I don't think they look that bad, so I don't really worry about it.
Replating has its' trade-offs with nickel as a jeweler that can do gold or silver might not have the gear to do nickel. There are spot plating methods, but like silver, these are thin.
Keep it clean, wipe it down after playing. Dirty nickel seems to get a slimy feel over time very easily.
I've never really dealt with a euph or tuba in nickel plate over longer periods of time.