I have read in this forum that gold plating can be preserved by putting clear nail polish over the keys it is on...
I assume there wouldn't be any harm in doing this on the headjoint? Would it change the your tone if you put a layer of clear nail polish on it?
Transparent lacquers and other similar protective layers are sometimes used to protect metals from losing the nice appearance they have when they are freshly polished. They could be used on flute keys, but I suggest no to do so.
There are two main reasons: one is that they are difficult to apply properly, as they are invisible for all practical means. When applying an invisible product, you could damage the pads or leave streaks of uncovered surface than soon much become darker that the protected part.
The second reason is that they are fragile and do not resists long term usage.
The result is that you get a metal piece where parts of the protection have worn out or peeled off and the metal quickly darkens there. It is very visible and IMO this is worst than having an uniform colour on that part, even if it was darker.
Once this happens, (and this could happen pretty soon), the only remedy its to remove it using a suitable solvent. The removal with solvent can be dangerous for the pad beneath the key, a risk that I believe is better to avoid.
My personal experience is that the keys are the parts of the instrument that remain without tarnish as the fingers are continuously wiping the tarnish off. This could be different in your case; I believe it has to do with the acidity of the skin that may vary for each person. I am lucky that my hands keep the instrument brilliant (where they can reach in normal use) and not the opposite.
This type of lacquers are very effective only on metal parts that are never manipulated and seldom touched; a typical example is a bronze hanging lamp. In such application the protective layer can last for many years.
A mess, sorry:
surface than soon much become darker that = surface that soon become much darker than
I am a little slow today; too early here.
I notice now you are talking only about gold plating of keys. As gold should not tarnish, my comments simply do not apply for your proposed application. Still I would not to it unless you find a suitable product that can resist the usage and is not too much dangerous to apply -and to remove. Nail polish does not seem to me to be fit for this.
I would not recommend coating gold plating with Nail polish. If Gold plating wears off, you get it re-plated. As far as tarnish is concerned, Gold plating on top of silver is pretty common and Tarnish can still form under the gold plating on the silver (for many reasons). It will take longer for the silver to tarnish though.
Most gold platings on intermediate instruments are little more than a thin flash plate. These don't last but can be replated.
I would not nail polish a whole headjoint. If you coat the whole tube it will change the way the tube interacts with the air column. You might find it beneficial, but you might find it horrible too. But change it, you will