Just two cents here - not an expert. Brass that is unlaquered oxidizes and tarnishes extremely easily, and touching it with your skin/hands will certainly do that. If it used to stay shiny, the laquer or other coating probably has worn off. No biggie.
Get some brass polish from a hardware store and clean it up. You may want to hit it with some very fine steel wool first to clean off any remaining laquer/coat for an even polish. If you want to keep some parts darker, be careful and finicky to only polish what you want to brighten up.
Fresh clear coat or a laquer when you're done would keep it looking shiny when done. Make sure all of the polishing compound has been completely wiped clean before you coat. Wear gloves when you polish to keep skin oils and salt off the material.
If you clear coat it as is, it will just be maintain the current tarnish, which doesn't sound like your goal.
Personally I like how brass weathers and patinas. Google brass finishing to find guidance from the internet freaks.
Ya if very thinly plated and a high use item you may have just worn through the pretty part. Then the paint/coat options might be better if you want it to look new again. Seems like a lot for a little, but choose your battles I guess. Good luck!